Authorities in Arizona used the term "domestic terrorism" to describe a series of freeway shootings, all of which occurred within 10 days along an 8-mile stretch of Interstate 10 in Phoenix, ABC News reported.

The latest of the incidents happened on Wednesday, and a $20,000 reward is being offered for information leading to an arrest, Col. Frank Milstead, who heads the Arizona Department of Public Safety, told the network.

"Ten days, nine incidents," he summarized. "This is a real and continuing threat to Arizona motorists."

Police have yet to determine if all the shootings are related, and they cannot be sure that each incident involved bullets, Milstead cautioned. But the officer minced no words when he spoke at a news conference on Tuesday, according to the Washington Post.

"Anytime you have multiple shootings against American citizens on a highway, that's terrorism," Milstead said. "They're trying to frighten or kill somebody. I don't know if this is a copycat crime, if it's multiple people that's involved in this type of insanity. Because somebody will get hurt, somebody will get killed. Don't kid yourselves. This is a very important matter for the department and the traveling public."

So far, no one has been seriously hurt in the shootings, most of which occurred in a stretch of the freeway that crosses the heart of the Arizona capital. In one incident, however, one bullet shattered a windshield and broken glass cut a 13-year-old girl, the Associated Press detailed. If more vehicles were affected, "it's just a matter of time before there is a tragedy," Milstead said.

Although the Department of Public Safety would not discuss the surveillance or other aspects of its investigation, spokesman Bart Graves admitted that police were not ruling out the possibility that more than one suspect was at work.

"We're not going to give the nuts and bolts of our investigation," Graves told the AP, explaining that doing so "would help the bad guy."