From left to right: Kevin Streine, Steve P. Guerra, Chad Smith and Alina Sanchez – the Firearm and Toolmark Unit at the New Mexico Department of Public Safety Forensic Laboratory in Santa Fe.
Several years ago, there was a triple homicide in Las Cruces. Almost exactly a year after the incident, a firearm was located in a chemical toilet at a highway rest stop. The firearm was brought to the New Mexico Department of Public Safety Forensic Laboratory Firearm and Toolmark Unit in Santa Fe where good fortune and forensic science collided to determine it was used in the crime.
Kevin Streine, a forensic scientist supervisor, was on the case and over a series of weeks he was able to clean the firearm enough to disassemble it into parts, clean again and reassemble, in order to make it function.
“The gun was in pretty rough shape. We were fortunate because the chamber was loaded with one round of live ammunition. This live round protected the chamber,” said Streine. “Absent that being the case, it likely would have deteriorated to the point where we wouldn’t have been able to make a determination.”
Image of the 9mm Luger caliber SWD brand semi-automatic pistol that was recovered from a chemical toilet at a rest stop.
He then shot the firearm into a water tank located in the lab in Santa Fe, recovered the test bullet and cartridge cases (“knowns”) and compared them to the evidence collected at the scene (“unknowns”). There was enough agreement of microscopic markings to conclude that the firearm recovered in the rest stop toilet was used in the triple homicide.
Cases like this illustrate how the Firearm and Toolmark Unit’s routine work can play a critical role in determining case outcomes and advancing public safety statewide.
“We analyze evidence and it all culminates in a court jury trial setting,” said Streine, a retired former New York City police officer and detective who has been working at the Santa Fe Forensic Laboratory for 23 years. “Everyone deserves to be safe, and our work contributes to that here in New Mexico.”
Kevin Streine, forensic scientist supervisor using the indoor range.
Alina Sanchez, criminal forensic scientist, who attended training at the National Firearm Examiner Academy (created by the Bureau of Alcohol, Tobacco, Firearms and Explosives) to kick off her career agreed: “One of the most satisfying parts for me is knowing my testimony was clear, understandable, and helped the jury make an informed decision. We are providing unbiased forensic support to law enforcement and to the judicial system in order to promote justice in our community, and I am passionate about doing that.”
Alina Sanchez, criminal forensic scientist looking through a microscope at the Forensic Laboratory.
Sometimes results lead to a bullet, cartridge case, or other evidence not being a match to a firearm that was recovered from a suspect.
“We can’t prove a person’s innocence, but we can potentially corroborate their story and say ‘this gun was not involved in this case,’ and often we don’t know the outcome but it’s satisfying knowing that you could potentially show someone’s innocence,” said Chad Smith, a five-year criminal forensic scientist in the unit with 20 years of experience as a forensic firearms examiner.
Chad Smith, criminal forensic scientist at the Forensic Laboratory.
“Kevin and I both came from law enforcement backgrounds,” said Steve P. Guerra, another criminal forensic scientist who is voluntarily certified in all three disciplines (firearms, tool marks, gunshot residue). “As a cop, you’re focused on protecting victims and catching the bad guys. I also worked crime scenes, trying to speak for the dead by finding evidence that explains what happened. But in the lab, you have to switch gears. Our job here is to interpret the evidence objectively, no matter where it leads, which could mean identification, elimination, inconclusive or unsuitable.”
Before comparing tiny, unique marks on bullets or cartridge cases, forensic scientists first check class characteristics – features set by the firearm’s manufacturer.
Steve P. Guerra, criminal forensic scientist at the laboratory in Santa Fe.
“For example, let’s say the evidence bullet is a 9mm Luger caliber bullet, but the gun they submitted is a .45 Auto caliber pistol. That is a class characteristic difference, very simply, the calibers are different. A 9mm Luger caliber cartridge is not typically fired in a .45 Auto caliber pistol,” said Guerra who has worked at the lab for almost 22 years and has decades of experience as a Firearm and Toolmark Examiner. “We also look at the rifling characteristics, which consist of lands (the raised portions between two grooves) and grooves, which we count. If our evidence bullet has six lands and grooves, but the gun that is submitted has eight, obviously we can eliminate the gun based on those class characteristics.”
“Millions of firearms can have the same class characteristics,” said Sanchez. “If we’re comparing a bullet from a crime scene to a test fired bullet (known sample fired in a submitted firearm), and the class characteristics are different, then we would eliminate it. However, if they are the same, that’s when we move on to the microscopic, individual characteristics, that are unique to each firearm, to determine if they are in sufficient agreement.”
The lands and grooves inside a gun barrel twist either to the right or left. This twist spins the bullet so it travels more steadily and accurately. As the bullet moves through the barrel, the lands and grooves leave impressions on it.
A fired cartridge case mounted in wax on a comparison microscope stage.
Analysts examine bullets and cartridge cases for tiny marks left by the firearm that fired them. They use a comparison microscope, which connects two microscopes so both objects can be viewed side by side in the same field of view. A test-fired bullet or cartridge case (known sample) is placed on one side, and the evidence sample is placed on the other. The analysts compare the marks to see if they match well enough to show that both were fired from the same firearm. They may also take photos of the comparison to show what they observed in court.
Identification of two fired cartridge cases based on aperture shear.
“Bullets usually only touch the inside of the gun barrel, so they can pick up microscopic marks from it,” said Streine. “Cartridge cases can collect marks from several parts of the gun, including the firing pin, extractor, ejector, breech face, and chamber.”
Fired cartridge cases that were fired in a Ruger brand pistol.
“No two firearms have ever been found to have the same patterns of individual (microscopic) characteristics,” said Sanchez who has worked at the lab for 22 years.
In addition to examining firearms, on occasion the unit also examines different types of hand tools that may have been used to cut or pry other metal objects. As an example, a pair of bolt cutters may have been used to cut off a padlock. Test cuts would be made in a softer metal (like lead) using the suspect bolt cutters. Those test cuts would then be compared to the cut padlock from the incident to determine if there is sufficient agreement of individual characteristics.
A sub-discipline of the unit is serial number restoration, which involves using chemical etching or magnetic particles in an attempt to restore a serial number that was obliterated from the surface of a firearm.
Alina Sanchez explains the serial number restoration process. She uses this poster for school presentations and for tours of the lab.
Another sub-discipline of the unit is muzzle-to-target distance determination. This allows the analysts to potentially determine how far the muzzle of a firearm was from the victim when they were shot. Muzzle-to-target distance determination could be useful in a case of homicide versus suicide. If a witness claims the victim committed suicide, but the shot was fired from a distance of 3-feet or greater, suicide is unlikely.
To work in the Firearm and Toolmark Unit, which completed over 200 cases in 2025, it takes patience, discipline and years of on-the-job formal training. According to Streine, for him a key motivator is helping others.
The “Firearm Reference Collection,” at the Forensic Laboratory in Santa Fe.
“When I became a Police Officer, one of my motivators was to help people,” said Streine. “It was in New York that I became involved with forensic science and the discipline of forensic firearm identification. My work as a forensic scientist is different from the work that I did as a police officer, but I do still feel that I’m helping people. Our job is to assist the men and women that work to solve cases using old fashioned investigative police work. They are the true crime solvers. We don’t have bias here; we go where the evidence takes us.”
Ammunition supply at the Forensic Laboratory.
The final member of the Firearm and Toolmark Unit who was unavailable for an interview due to having to go to court is Matt Bustamante who is certified in firearms and toolmarks and awaiting a final test in gunshot residue – hoping to join Guerra as one of 27 forensic scientists in the world certified in all three disciplines.
Story by New Mexico Department of Public Safety Public Information Officer John Heil. Photos by DPS Media Relations Unit – Payton Santillanes.