Authorities in Arizona are treating a surge of shootings that occured on the I-10 as an act of domestic terrorism.

Since Aug. 29, nine shootings have occured along an eight-mile stretch of the I-10, leaving drivers terrified, reported the Star Tribune. Last week there were four shootings: one bullet shattered a windshield and cut a 13-year-old girl, while the others hit a headlight and sides of vehicles. Then on Tuesday, police announced five more shootings, including one that shattered the window of an off-duty police sergeant.

Arizona Director of Public Safety Colonel Frank Milstead revealed that a $20,000 reward is being offered for information leading to the arrest and conviction of the shooter or shooters involved in the spree, reported ABC News. Before Tuesday, that reward stood at $5,000.

"Any time you have multiple shootings against American citizens on a highway, that's terrorism. They're trying to frighten or kill somebody," he said.

He revealed that authorities are investigating all nine shootings but refrained from saying that there was serial shooter, since police don't know whether all nine shootings are connected or if a copycat might be at work, according to NBC News. 

Milstead said drivers have been lucky that no one has been seriously injured or killed, but warned, "somebody will get hurt or somebody will get killed if this continues. Right now there is a real and continuing threat to Arizona motorists."

Agents with the FBI and ATF have joined the investigation, and drivers in the area have been urged to "stay alert".