By Jonathan Drew
The Associated Press

COLUMBUS - Though a sniper's bullet hits its target in an instant, it can take forensic experts days or weeks to match the projectile to its firearm or another bullet.

Microscopes will magnify the bullets and computer software can indicate similarities between digital images, but links must be verified by a human investigator comparing hundreds of tiny scratches and taking notes by hand.

It has been more than six weeks since a 62-year-old woman was shot and killed while a passenger in a car traveling on Interstate 270 on the city's south side. It is one of 18 shootings in the area over the last several months that has been linked by authorities.

A task force has connected seven recovered bullets from the shootings, and investigators have interviewed hundreds of people without identifying a suspect.

No weapons have been recovered from any of the shootings, and the task force won't say how many weapons it has tested from those who have been interviewed.

The time-consuming process of discerning a bullet's "fingerprint" sheds light on why the probe has dragged on. Experts say forensic testing of a bullet can take anywhere from a few hours to more than a week.

At the Columbus Police Crime Lab, which has handled all ballistics testing from the investigation, only one analyst is trained to compare bullets, said Jami St. Clair, the lab manager.

The analyst had to give up vacation time last year, and the testing of bullets in other cases has lagged because of the investigation, said Sgt. Brent Mull, a police spokesman.

"He's been working seven days week," St. Clair said. "I think he took Christmas off; that's about it."

Each bullet fired from a gun retains a pattern of scratches from imperfections in the gun's barrel. The unique patterns can be used to match bullets to one another or to a specific firearm.

Through a powerful microscope, the surface of a tiny, round bullet appears flat and furrowed - like a metal washboard but more haphazard.

The process begins well before the bullet reaches the lab. Two days after a house was shot on Dec. 15 - one of the 18 shootings to be connected - a technician on a ladder spent more than an hour carefully examining two bullet holes. Gusts of frigid air blew across the front yard as he used pliers to remove siding around one of the quarter-sized holes and later chipped at the wood beneath with a hammer and chisel.

Such deliberate work is required so the bullet isn't damaged further.