The FBI has joined state and local law enforcement investigating reports that at least nine vehicles were hit by gunfire in the days following a deadly series of highway shootings in southern Indiana.

In the earlier shootings, a teenager was quickly arrested and has pleaded not guilty to murder and attempted murder.

The latest string of shootings started two days later, on July 25, as the vehicles were driving on major roads in heavily populated northeast Indiana. Authorities believe all nine shootings are related and more than one person might be involved.

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"We believe at this point that we need to take every measure in this area and utilize every resource possible to apprehend this person and protect our citizens," Lake County Sheriff Roy Dominguez said Monday.

Authorities investigating the damaged vehicles have not found slugs or shell casings, but Dominguez said officials believe the damage was caused by some sort of firearm or pellet gun.

Mark Becker, supervisory agent for the FBI's Merrillville office, said the agency is working on a profile of the suspect or suspects.

"We certainly believe the person is unstable," he said. "No one in their right mind would try to create this kind of havoc."

Eight vehicles were hit on a roadway between Hammond and Gary, now being monitored by police in cars and helicopters. The ninth was damaged on the Indiana Toll Road. Dominguez said all had been isolated from other traffic when they were hit.

In the earlier shootings, carried out July 23, 17-year-old Zachariah Blanton is accused of killing a truck passenger and wounding another on Interstate 65 near Seymour, then shooting two unoccupied vehicles a few hours later near Muncie, not far from his home. He has pleaded not guilty.

The shootings were also a reminder of attacks in neighboring Ohio, where in late 2003 and early 2004, a sniper in the Columbus area killed one person in a series of random highway shootings. Charles McCoy Jr. later pleaded guilty and was sentenced to 27 years in prison.