Drivers Distressed Over Possible Freeway Shootings in Arizona

— -- Authorities are investigating a series of possible shootings within 10 days along an 8-mile stretch of the I-10 freeway in Phoenix.

A 10th incident was reported Wednesday, Colonel Frank Milstead, director of the Arizona Department of Public Safety, told ABC News. Milstead said it's unclear whether all of the incidents are related and also unclear whether all incidents involved bullets.

Milstead classified the incidents as domestic terrorism. He called the suspect or suspects cowards who are putting innocent lives at risk.

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

Agents with the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives 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.

The Associated Press contributed to this report.