By DAN GOOD via Good Morning America

Drivers along a highway in Phoenix have endured nine possible shootings in the last 10 days in a potential act of domestic terrorism, authorities said.

The freeway shootings have occurred along an eight-mile stretch of I-10.

The spree began Aug. 29, when a bullet shattered the windshield of an SUV, injuring a 13-year-old girl, authorities said. Another shot hit a tour bus, putting holes through the seats. Robert McDonald was driving the bus at the time of the attack and was the only person on board.

“The person almost took me away from my family and almost caused catastrophic injuries to the other motorists on the street,” McDonald said.

Two more shooting incidents were reported Tuesday, including one involving a police sergeant’s personal vehicle.

“Both vehicles were hit on the driver side, passenger window by a projectile,” Arizona Department of Public Safety Director Frank Milstead said. “I would call that related.”

Agents with the FBI and ATF have joined the investigation, and drivers in the area are warned to be vigilant.

The reward for information leading to the arrest and conviction of the shooter or shooters was increased to $20,000 Tuesday, according to the Associated Press.