The shootings along a busy stretch of Interstate 10 in Phoenix started Aug. 29, when a bullet from out of nowhere struck an SUV, shattering glass, which injured 13-year old girl riding in it. On that same day, along the same stretch of road, someone fired at a bus, fortunately empty except for the driver, who was not injured.

But bullet holes were found in some of the seats of the bus. That night, about 10, yet another car was hit, again, with no injuries.

By Tuesday this week, there had been 6 more similar incidents and no arrests or suspects, prompting Col. Frank Milstead of the Arizona Department of Public Safety to hold a news conference calling the attacks “domestic terrorism” and offering a $20,000 reward for information leading to the arrest and conviction of the “person or persons” responsible.

The response came the next morning: Another reported shooting at a vehicle on I-10, in that same 8-mile stretch. Again, nobody hurt when a white truck on I-10 was struck.

While the police press release described “ten shooting incidents,” they seemed uncertain whether all involved bullets, describing some of the vehicles as being hit by “projectiles,” suggesting that they had not recovered in all cases whatever it was that made the holes.

“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,” Milstead said.

Milstead declined to compare Arizona’s situation to the so-called Washington Beltway sniper attacks of 2002, which took the lives of 10 people before John Allen Muhammad and Lee Boyd Malvo were apprehended.

Those shootings took place over a much wider area and were clearly targeted not simply at vehicles but specifically at people, standing outside of stores or service stations or mowing a lawn. Milstead believes that “a number of different weapons” have been used, unlike the Beltway sniper shootings.

Nor are police positive they are all connected, though some clearly were, said Milstead, like the two that were separated by one minute Tuesday, the first at 5.20 a.m. when an eastbound vehicle as hit and a second in the same area at 5.21 a.m., when a westbound vehicle was struck.

Asked what advice he could give to frightened motorists, Milstead said: “If you drive that stretch of roadway, always be vigilant. Under these circumstances be hyper-vigilant.”

He pleaded for information from anyone who had seen or heard anything or noticed suspicious behavior.

“The other thing we always see in these type of domestic terrorism type of crimes,” Milstead said, “is that somebody knew something …  and they say ‘you know what? I thought that guy was suspicious. I thought that guy was having problems.’ And they didn’t say anything.”

Authorities have mobilized a task force including local SWAT teams, the FBI, state and local police and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives in an effort to find out who’s doing the shooting and why and put an end to it.