(Photo courtesy of Arizona Department of Public Safety)

There have been a series of shootings along Interstate 10 in Phoenix.

(Photo by Sky Schaudt - KJZZ)

A Phoenix freeway sign asks drivers to report suspicious activity.

The Arizona Department of Public Safety is investigating nine cases where vehicles were hit with projectiles on Interstate 10. Two more car shootings took place Tuesday along Interstate 10 in Phoenix, but it's unclear if they're related to four other incidents of shots being fired at moving vehicles on the freeway since late last month.

DPS also is investigating three other shootings of bullets or projectiles at vehicles in recent days on the Loop 202 freeway, State Route 51 and another Phoenix-area highway but details weren't immediately released.

DPS announced Tuesday this investigation is its top priority and will partner with federal law enforcement agencies.

Colonel Frank Milstead called this domestic terrorism.

“Somebody will get hurt or somebody will get killed if this continues," Milstead said. "We are still having vehicles shot at on Arizona roadways.”

No drivers have been seriously injured. DPS is offering a $20,000 reward for information leading to the arrest and conviction of those involved in the crime. 

Call 602-644-5805 with tips or information on the incidents. 

The Associated Press contributed to this report.