Microscopic clues on bullets and fragments have allowed investigators to link shootings carried out by a deadly sniper terrorizing the Washington area, but authorities are still trying to pinpoint the gun used.
After analyzing ballistics and other evidence, law enforcement officials concluded Wednesday that the most recent shooting that killed bus driver Conrad Johnson in Aspen Hill, Md., on Tuesday was indeed linked to the sniper, said Michael Bouchard, an agent with the federal bureau of Alcohol, Tobacco and Firearms.
That means the sniper has killed 10 people and critically wounded three others in Maryland, Virginia and Washington since Oct. 2.
While ballistics evidence has revealed that a .223 caliber bullet was fired from the same rifle, narrowing down the exact make and model of the gun used is tricky.
"Upwards of 100 different rifles" use that ammunition, said ATF spokesman Jim Crandall. All kinds of rifles use a .223 caliber bullet, including guns used in sporting activities, military weapons, such as the M-16 rifle, and some "assault type of weapons," Crandall said.
Jim Kouri, vice president of the National Association of Chiefs of Police, said that the .223 caliber bullet tends to break up more than other bullets upon impact, a challenge for ballistics examiners. "It is very very difficult to make comparisons when you get a fractured or shattered round," Kouri said.
Kouri also said that the .223 caliber bullet holds more gunpowder and travels three times faster than the popular .22 caliber used in many handguns and rifles.
"I don't believe the sniper fires more than once," Kouri said. "He doesn't want any bullets out there in good condition."
Every firearm has individual characteristics that are as unique to it as fingerprints are to human beings, ATF says.
However, an ATF database that helps firearm examiners match ballistics evidence contains only images of casings or bullets from crime scenes or from test firings of weapons used in crimes. It doesn't include images of every gun sold in the United States, so a gun that hasn't been used in a previous crime isn't included in the computer database.
Firearms examiners analyze microscopic scratches and dents — on the bullets, their fragments or cartridge casings — to try to determine whether they were fired from the same weapon.
Grooves inside the barrel of a gun help a bullet travel with precision. When a bullet is fired, these grooves and other unique characteristics are imprinted on the bullet. When a bullet or bullet fragment is recovered from a crime, it is examined to see if a pattern of grooves and "lands" — the distance between the grooves — can help determine the type of firearm that was used. Examiners also weigh the bullet or bullet fragments to try to identify the caliber and type of firearm.
Bullet casings also can have distinctive markings created by the gun's firing pin, ejector and breech mechanism — the place where the bullet sits in the barrel of the gun. Sometimes a fragment can be too small, making any markings all but impossible to divine.
Other factors make work difficult for firearms examiners.
"Factors related to uniqueness that can complicate the identification process include the presence of `subclass characteristics,' or markings, common to groups of firearms or ammunition, that can be mistaken by machine or examiner for markings individual to a firearm," ATF said in a May 13 report.
It is possible for a person, using a file or some other object, to try and alter a firearm so that the bullets and cartridge casings fired from it would have a different appearance, ATF said. While that wouldn't be difficult or time-consuming, such instances of this occurring in actual casework are exceedingly rare, the ATF report said.
"Because of the microscopic character of the changes, it is not possible to alter one firearm in order to make the imprint look like another," the ATF report said. "Rather, the idea of altering a firearm would be to prevent a definitive identification by creating additional markings for examiners or automated equipment to read."
In some cases, the buildup of dirt and debris can have "minor impact on the markings made on ammunitions, though this does not necessarily lessen the markings and can in fact magnify them," the report said.
Kouri said the process of trying to use ballistics information to find the gun to track down the sniper is tedious work. "People are used to TV shows in which the bad guys are apprehended in an hour. That's not how it works."