Arizona Department of Public Safety officials were investigating three 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 two other shootings of bullets or projectiles at vehicles in recent days on the Loop 202 freeway and State Route 51, 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."

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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.

Governor Doug Ducey issued the following statement Tuesday:

"My office has been in constant contact with the Department of Public Safety, and I am receiving regular updates from Colonel Milstead. The safety of Arizonans is our No. 1 priority, and we are committed to apprehending those responsible for these crimes. I ask all Arizonans to stay alert and contact authorities with any information that could lead to the arrest and prosecution of responsible parties. I assure you, this is the top the priority for the Department of Public Safety and the state of Arizona."

Tipsters can remain anonymous. Silent Witness can be contacted at 480-WITNESS or you can call the DPS tip line at 602-644-5805.