COLUMBUS, Ohio (AP) – A bullet hole found in a van has been linked to a string of shootings along a stretch of interstate, bringing the total number of cases under investigation to 15, authorities said Tuesday.

The shooting at a used car lot on U.S. Highway 23 was noticed Nov. 15, 10 days before a woman was killed while driving on nearby Interstate 270, where most of the shootings happened. Many of the victims called law enforcement agencies for the first time after hearing of the fatal shooting Nov. 25.

The van incident was the sixth shooting police have connected through ballistic tests, Chief Sheriff’s Deputy Steve Martin said. Although the rest of the shootings could not be linked through bullet testing, police believe they, too, are related.

On Monday, workers put up cameras on I-270 in an attempt to find who is behind the shootings.

A passer-by reported a hole in the van to the car lot owner, police said. When police initially investigated the hole, they gave the car lot owner the bullet, which was found on the ground. After the shootings started happening, the lot owner returned the bullet to investigators, Martin said.

At least two shootings along I-270 have happened since Nov. 25, when Gail Knisley, 62, was killed as she was being driven to a doctor’s appointment. She is the only person hit in the shootings.

Police have received 1,520 tips about the shootings. They asked people to compare the times and dates of the shootings with the habits of individuals they suspect may be involved.

Police are offering a $20,000 award for information leading to an arrest and charges.