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Identifying Firearms

Forensics Class

Christopher Ervin

on 12 December 2014

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Transcript of Identifying Firearms

Identifying Firearms, Identifying Individuals, Evidence From Blood and DNA
Vista Grande High School
Mr. Ervin

Searching for Blood
Evidence From blood
Blood is a water based mixture, with cells suspended in the water. The image shows a type of cell found in blood, red blood cells.
Red blood cells consist of mainly hemoglobin, the molecule that carries oxygen to cells in the body. Some chemicals produce light or change color in the presence of hemoglobin. The chemicals can be used to detect blood at a crime scene.
Finding Blood
Evidence From Blood
A CSI can use UV light to find blood at a crime scene. If this method fails, they can spray a chemical called luminol on surfaces where he suspects there is blood.
Luminol emits a blue glow when it comes in contact with blood. The glow lasts about 30 seconds, which is long enough to photograph the evidence.
Luminol can be used to search large areas quickly. It also detect the small traces of blood left behind when someone tried to clean up the crime scene. Luminol does have one draw back: Blood is not the only material that can cause luminol to glow. So a positive test with luminol may be a false alarm.
Testing Blood
Evidence From Blood
ABO Blood Groups
Classifying Blood
Stains that look like blood may not actually be blood. A CSI can run an initial test on a stain at the crime scene. They can use a moist cotton swab to take a sample of the stain. Then they can rub the swab against a strip that contains chemicals that change color in the presence of blood.
There are other materials that may also yield those same results, horseradish, can produce the same results. Stuff like horseradish, are rarely found at the scene of a crime. So a positive test is usually a reliable first test for blood. Once back in the lab another test will be done to confirm the initial results.
After a conformation has been done, scientists can test to see if the blood is human blood. At this point they may also do some inexpensive tests to narrow the list of possible suspects.
In the eary 1900's, doctors were looking for a way to save patients who had lost large amounts of blood. They tried to transfer blood from a healthy donor to the patient. Usually those transfers didn't work and one doctor, Karl Landsteiner, was able to figure out why.
He discovered that human blood can be classified into 4 groups, or blood types. These are A, B, AB, and O.The groups are named after marker molecules found on the surface of red blood cells.
ABO, Continued
Classifying Blood
Some people have only the A molecules. Some have only the B molecules. People with AB blood have both A and B molecules. People with type O blood have neither A nor B molecules.
Classifying Blood
Marker molecules act as tags. The tags tell whether the blood is part of your body or not.
Antibodies are molecules that bind to marker molecules. Each antibody will bind to one specific molecule. Scientists can use antibodies to classify blood.
Using Blood Types as Evidence
Blood types are not like fingerprints. They can not be used to identify a suspect. But they can point investigators in the right direction.
Using Blood Types as Evidence, Cont.
Classifying Blood
For example, police find a blood stain on a suspect's shirt. "I cut myself shaving," the suspect explains. But the suspect has type A blood and the stain on his shirt is type O. The police will wonder why the suspect lied to them. And, if the victims blood is type O, the police will be even more suspicious.
Knowing the blood type can reduce the number of suspects. There are about 250 possible marker molecules for red blood cells. If scientists had a larger blood sample and enough time, they might be able to use blood types to identify a suspect. But scientists now have a better option. They can use the blood to do DNA tests.
Distance and Angle of Impact
Bloodstain Patterns
Investigators may find drops, splashes, or pools of blood at a crime scene. Each stain tells a story. Did blood drip from a cut? Did blood spurt from an artery? Did a victim leave smears of blood while crawling across a floor?
Investigators analyze patterns of bloodstains to figure out what happend at a crime scene.
The number and location of bloodstains is important. So is the shape of the bloodstains
Distance and Angle of Impact
Bloodstain Patterns
Blood that falls a short distance usually leaves a round stain on a hard surface.
If the surface is soft or porous, there is likely to be spatter and the stain will have ragged edges.
Which of these bloodstains fell on a hard, smooth surface at a 90 degree angle?
Size of Drops
Blood that is sent flying through the air by a blow will break into smaller drops when it hits a surface. Investigators can estimate the force of the blow by the size of the drops. Generally, as the force increases, the size of the drops decrease. So, force and drop size are inversely proportional.
Direction of Travel
Bloodstain Patterns
An analyst can also use the shape of bloodstains to figure out the location of the attack. They know that the tip of a stain always points in the direction that the blood was moving.
If there are multiple stain on a surface, they can draw a line through the long axis of each stain. Then, they can follow the lines back to the point where they meet. This is the spot where the blood was released.
This used to be done by hand, but now all of this can be done with computers. The computer programs can accurately plot the path that the blood traveled from its source.
Absence of Stains
Bloodstain Patterns
There may be space in the middle of a blood spatter pattern where there is no blood. This lack of blood can be an important clue. Investigators will suspect that there was an object in that location at the same time as the attack. The object had to have been removed for one reason or other. If investigators find the object, they are likely to find the rest of the blood.
DNA Evidence
DNA Molecules
Do you have curly hair? Brown eyes? These characteristics are traits that are carried from parents to offspring by DNA, or Deoxyribonucleic acid. DNA controls the production of proteins in the human body.
are the molecules that your body uses to build tissues and organs. Proteins also control the chemical reactions that take place in cells. DNA is found in the nucleus, or control center, of body cells. This nuclear DNA is a combination of the DNA you inherit from each of your parents. It is the same in every body cell - hair, skin, muscle, and so on.
DNA Evidence
Structure of DNA
The figure at right shows the structure of a DNA molecule. Two long strands of DNA coiled around one another. Weak chemical bonds between pairs of nitrogen bases connect the strands. The letter A, T, G. and C are used to represent these pairs of bases, or pairs. Except for identical twins, no two people have nuclear DNA with the exact same sequence of base pairs.
DNA strands are divided into sections. A gene is a section of DNA that contains information your cells need to make protein. The order of the bases in a gene is a code that determines which protein is produced. Humans have about 24,000 genes with a total of three billion base pairs.
You might expect that a strand of DNA is filled with genes. But this isn't true. Some sections of DNA are non-coding, that is, they do not contain instructions for making protein. In non-coding DNA, a sequence of bases can be repeated many times. The number of times a sequence repeats is an inherited trait. The number of repeats for a sequence can vary. Scientists use this variation to make DNA profiles.

DNA Profile
is a distinctive pattern of DNA fragments. The pattern is used to match a biological sample to an individual. To make a profile, scientists must collect, isolate, multiply, and sort DNA.
Making a DNA Profile
Collecting DNA
A CSI may find a suspect's blood or skin under a victim's fingernails. There could be Saliva on a pillow or a licked stamp. A discarded facial tissue might contain a suspect's sweat or blood. All of these materials contain DNA. Investigators may use a swab to take cells from the inside of a suspect's cheek. They can also find DNA on a toothbrush or a comb. A CSI must be careful to avoid contamination when collecting DNA samples for analysis. Tools, such as a swab box can help.
Isolating DNA
Making a DNA Profile
DNA must be removed from a biological sample before it can be tested. Suppose a scientist has a sample of blood. They use a microfuge like the one below to spin the blood at high speed.
As the blood spins, the cells seperate from the liquid. Then they add chemicals to release the DNA from the white blood cells. Finally, they add alcohol and a sticky blob of DNA settles out of the mixture.
Multiplying DNA
To understand what happens next, you need to know how a DNA molecules make a copy of itself. This process is called replication. The bonds between the base pairs break. The strands begin to seperate and unwind, like a zipper unzipping. Nitrogen bases in the nucleous bond to the bases on the single strands. The bases are added according to the rule that A always bonds with T and C always bonds with G. As the bases are added, the original molecule and the copy rewind, as shown here.
Multiplying, continued.
Making a DNA Profile
Trace evidence often does not contain enough DNA to make a profile. A forensic scientist can use DNA replication to increase the amount of DNA. The scientist starts with the DNA that was isolated. They do not copy all of the DNA, They copy only 13 segments of non-coding DNA,
Each time the segments are copied, the amount of DNA doubles. The process can be repeated again and again. In just a few hours, a scientist can repeat the process about 30 times. If they do, they will have about a billion times as much DNA as they started with.
Sorting DNA
The result of the DNA "multiplication" is a mass of DNA fragments. These fragments need to be sorted to produce a virtual profile. In one method, an electric field pulls the fragments through a thick gel. The fragments travel at different speeds depending on their length. The shorter the fragments move more quickly than the longer fragments. The process produces a pattern of bands like the one shown here.
Some people refer to a DNA profile as a DNA "fingerprint." Most forensic scientists do not do that. They don't like to imply that a profile is unique the way a fingerprint is. It is possible that two people could have the same profile. It just isn't probable.
is a measure of the chance that an event will happen. When you toss a coin, it can land heads up or tails up. The probability of either event is 1 in 2 or 50%. Suppose you select any 2 people in the world. Scientists can calculate the probability of their having the same number of repeats in a given DNA segment.
Scientists often test at least 13 different segments. As the number of segments tested increases, the probability of two people having the same DNA profile decreases. What is the probability when 13 segments are tested? It is less than 1 in 500 trillion (500,000,000,000,000). With odds like this, courts are likely to accept DNA evidence.
Uses of DNA Profiles
DNA is rarely the only evidence in a case. But it is often the most persuasive evidence. DNA profiles are used to connect a suspect to a crime. They also help solve cold cases, free the innocent, identify human remains, and protect endangered species.
Connecting a suspect to a crime
Uses of DNA Profiles
Newer methods for making DNA profiles are faster than earlier methods. But they are still expensive. So DNA profiles are used most often to solve very serious crimes. If police have a suspect, the lab can compare a profile of the suspect's DNA to one prepared with evidence from the crime scene.
Police can also search for a suspect in a DNA database. Every state has a DNA database. These databases store profiles of offenders who were convicted certain violent crimes. The profiles are stored as a series of numbers. So finding a match is a fairly quick process, if a match exists. The FBI keeps a national database called CODIS. CODIS stands for Combined DNA Index System. It has data from the 50 states, the armed forces, and the FBI. CODIS gives every crime lab access to move data. CODIS includes profiles of known offenders and missing persons. It also has data from forensic evidence.
Solving Cold Cases
Uses of DNA Profiles
Not every case gets solved. The police may not have enough evidence or any good leads. If a case isn't solved within a year, the case may be filed away. Old, unsolved cases are known as
Cold Cases.
Many police departments have set up special units to look into cold cases.
Some crimes happen before DNA testing existed. Suppose police have biological evidence from a cold case. They can send the evidence to a lab and ask for a DNA profile. Adding the profile to the database could help solve the crime. In Ohio, for instance, a man was sent to prison for robbery. His DNA profile went into the database. It matched the profile of DNA found at the scene of an unsolved murder.
Freeing the Innocent
Uses of DNA Profiles
Many people who are in jail claim that they are innocent. Most are guilty of the crime for which they are being punished. Some, however, are innocent.
In 1992, some lawyers in New York City set up a free legal service. Its goal was to defend prisoners whose claims of innocence could be proven by DNA testing. The idea spread. Law students, journalism students, and lawyers who defend poor clients set up similar services in other states.
Freeing the Innocent, Cont.
Sometimes a DNA test shows a prisoner's claim of innocence are false. But many times, the evidence has helped to free innocent people. In 1997, for instance, Ryan Matthews was accused of shooting a man during a robbery. He was found guilty and sentenced to death. In 2004, Matthews was released from jail. The key was DNA found on the inside of a ski mask left at the crime scene. The DNA profile matched the DNA of another man.
Identifying Human Remains
Most DNA tests use nuclear DNA. But sometimes it is not possible to get DNA from the nucleus. Some cells don't have a nucleus. Also, nuclear DNA can be damaged when a body decays or is burned.
To identify human remains, scientists can turn to DNA that is found outside the nucleus. This DNA, which is found in all cells, comes only from a person's mother. It is more abundant than nuclear DNA and is less likely to decay. This method was used to identify the bones of Tsar Nicholas II.
Protecting Endangered Species
Uses of DNA profiles
DNA is not just used to identify human beings. It can be used to identify other species. Some species have been classified as endangered. An
Endangered Species
is a species whose numbers are so small that the species may disappear from the world.
Some people work hard to protect endangered species. But other people try to use them to make a profit. They may sell protected animals to restaurants. Or they sell specific parts of an animal's body - an elephant's tusk or the skin from a tiger. Some people will buy rare animals, especially birds, to keep as pets.
The million doillar question is, how can DNA help? In Florida, scientists were able to show that the "tuna" on some restuarant menus was really sailfish. Sailfish is protected by federal law. Tests also showed that whale meat being sold in Japan came from species that were protected by international law.
Athletes in the Olympics take part in an event called the biathlon. The biathlon involves a skier and the skier shooting a rifle at various targets.
Not every use of a rifle is as innocent as in a biathlon.
Many crimes in the United States invovle the use of rifles and handguns. Forensic scientists have the task of identifying the firearms used in those crimes.
Identifying Firearms
Evidence From Firearms
Crime scene investigators may find a gun or a person with a gun at the scene of a shooting. But this doesn't happen in most cases. Investigators must use the evidence left behind after a weapon is fired.
How Firearms Work
Firearms are designed to shoot an object at high speed toward a target. In a pistol like the one below that object is the bullet. Each bullet is packed in a case, or cartridge. The cartridge also contains gunpowder and a primer.
When the trigger is pulled the firing pin strikes the cartridge. The pressure causes the primer to ignite, which in turn ignites the gunpowder. The hot gases produced by the reaction push the bullet through the barrel of the gun at high speed.
Evidence From Cartridge Cases
Evidence From Firearms
A CSI can learn many things by just observing empty cartridge cases. The size of the cases can help narrow down the list of possible firearms. When cases are found, they are typically near the spot where a weapon was fired. From the number of cases, the CSI can infer how many bullets were fired.
How Firearms Work
Firing a gun leaves impressions on a cartridge case. The firing pin leaves a small, distinctive dent. Marks are also stamped on the case when it is pressed against the inside of the firing chamber and when it is ejected from the gun.
First investigators must find the bullets, X-rays are used to locate bullets in shooting victims. If the victim is alive, the bullets may be removed during an operation. If the victim is dead, the medical examiner will retrieve the bullets during autopsy.
Evidence from Bullets
If a bullet misses the victim, the CSI can search for that bullet. Once the CSI finds the bullet they can use the location of the bullet to trace its path back to the spot where the gun was fired.
A CSI uses the marks on the bullet as evidence. Firing a gun leaves an impression on the bullet. Inside the gun barrel, there are spiral grooves called rifling. Those grooves cause the bullet to spin as it passes through the barrel. Rifling leaves marks on the bullet that match the size, spacing, and angle of the grooves.
The rifling on any barrel help to narrow down the number of possible guns that could have fired the bullet. Gun barrels also, through normal use, get nicks and bumps, and scratches on them. These also leaves marks on a bullet that will help CSIs connect it to a certain gun.
Impressions are not the only physical evidence produced when a gun is fired. When a gun is fired it leaves trace evidence on the person that fires it. A gunpowder residue is left on the person firing the gun.
Gunshot Residue
is the unused gunpowder and primer left over after the gun is fired. Often time there is gunpowder and part of the primer in the bullet that is not burned during the shooting of a gun. This residue is expelled out of the barrel and out of the trigger hole of the gun when it is fired. This unused gunpowder and primer can end up on the shooter's hands, face, and clothes.
Evidence From Gunpowder
A CSI may use a swab or tape to collect the trace evidence left behind from firing a gun.
Evidence From Gunpowder
Trace evidence left behind when firing a gun is easily washed off and destroyed. So a CSI will often look for traces on a person's clothes.
Analyzing Firearms Evidence
A firearms analyst wants to identify the weapon used in a crime and the person who fired the weapon. Microscopes and chemical tests are used to check for gunshot residue. Microscopes and databases are used to compare impressions from firearms.
Testing for Gunshot Residue
Analyzing Firearms Evidence
Some particles in gunshot residue have distinctive shapes. When the particles are magnified with a scanning eletron microscope, the shape is easy to see. Gunshot residue contains traces of chemical elements lead, barium, and antimony. A CSI uses chemical tests to identify the elements present. A CSI will use a swab to collect residue that will be chemically tested and tape will be used to collect evidence that will be magnified with the scanning electron microscope.
Testing a Firearm
To figure out if a gun was used in a crime, an analyst needs a bullet to compare with one from the crime scene. The CSI will fire the recovered gun to get a bullet to match to the one recovered at the crime scene. The bullet will be fired into water or a ballistic gel so it is not damaged.
The analyst compares the rifling on the test bullet with a bullet from the crime scene. If the rifling matches, they have the right type of gun. To find out if it is the actual gun used in the crime a CSI must examine other features associated with the bullet and the gun.
Testing a Firearm
Analyzing Firearms Evidence
Using a comparison microscope, an analyst can see the two bullets at the same time, they can compare the two bullets to see if they contain the same pattern and scratches. If the two bullets have the same patterns on them, the analyst has proof that the bullet from the crime scene was fired from the gun being tested.
The microscope is also used to compare marks left on the cartridge cases, as shown below. Those marks can also help match a specific gun to a crime.
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