Arizona Department of Public Safety officials were investigating two more car shootings 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.

"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."

The state Silent Witness program quadrupled the reward Tuesday to $20,000 for information leading to the arrest and conviction of the shooter or shooters.

There's been one reported injury — a 13-year-old girl who was cut by shattered glass.

But Milstead said that if the incidents continue, "it's just a matter of time before there is a tragedy."

Authorities have been asking for the public's help in tracking down suspects in the incidents.

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

The incidents began in the late morning of Aug. 29 when two vehicles were struck in a half-hour span on I-10 between 19th and 59th avenues and a third vehicle was hit on I-10 near 16th Street late that night.

A fourth vehicle was shot on Aug. 31 around 4:25 a.m. near 14th Avenue and I-10.

On Tuesday, a Phoenix police sergeant's personal vehicle had a passenger window shattered while he driving to work around 5:20 a.m. on eastbound I-10 between 35th and 43rd avenues, according to DPS officials who said the sergeant wasn't injured.

DPS didn't immediately provide details on the other Tuesday incident, but Milstead said it occurred at about the same time in the same area.