Who’s shooting at cars on Phoenix freeways?

That’s what Arizona officials are trying to find out as they quadruple a state reward for information leading to the arrest of the shooters from $5,000 to $20,000.

The Department of Public Safety made the appeal Tuesday as it announced two more car shootings in the ongoing spate that’s unfolded in the past two weeks. Both had taken place on the Interstate 10, though it’s unclear whether they were related to similar incidents on other freeways, including the Loop 202 and State Route 51, said officials.

“This is a priority for our department,” DPS Director Frank Milstead said at a news conference. “Ten days, nine incidents. This is a real and continuing threat to Arizona motorists.”

While only one injury has been reported so far – a 13-year-old girl with a minor cut from broken glass – Colonel Milstead said “it’s just a matter of time before there is a tragedy” if the shootings continue, according to the Associated Press.

“I don't know if this is a copy-cat crime or multiple people partaking in this kind of insanity,” he said.

The incidents began in the late morning of Aug. 29. In the span of half an hour, two vehicles were struck on I-10, authorities said. Late that night, a third car was hit on the same freeway.

Two days later, a fourth vehicle was shot in the middle of the night near the I-10.

On Tuesday, a Phoenix police sergeant’s personal vehicle was struck as he drove to work on the I-10 around 5:20 a.m., according to DPS. A passenger window was shattered, though it wasn’t clear whether the car had been struck by gunfire or something else, such as road debris.

Officials said the sergeant, who was not in uniform at the time of the shooting, wasn’t injured.

Branding the recent incidents “domestic terrorism” and urging motorists to stay alert, Milstead said state police was using its gang task force and special weapons and tactics personal and undercover detectives for freeway surveillance. Both federal and local law enforcement officials are investigating.

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“If you drive that stretch of roadway, always be vigilant,” he said. “Under these circumstances, be hyper vigilant.”

This report contains material from the Associated Press and Reuters.