The number of possible vehicle shootings along a stretch of freeway in Phoenix has jumped to nine over the last 10 days, the head of Arizona's state police said on Tuesday.
Colonel Frank Milstead, director of the Arizona Department of Public Safety, said the attacks continued following two possible shootings early Tuesday on Interstate 10, the southernmost transcontinental highway in the U.S.
The shootings began on Aug. 29 along a roughly 10-mile segment of the highway when gunfire shattered the windshield of a sport utility vehicle, authorities said.
A 13-year-old girl sustained a minor cut to her right ear, the only injury that has been reported.
On Tuesday, a Phoenix police sergeant told state troopers he believes a bullet shattered the passenger side window of his personal vehicle as he was driving to work.
See photos of bullet holes in windshields:
Shootings along Arizona freeway
Arizona police report nine possible shootings along interstate highway
Arizona Department of Transportation Live Traffic Operations Operators Lisa Supplee, rear, and Chuck Wood monitor over 200 freeway camera's throughout the Phoenix Metro area, Thursday, Sept. 10, 2015 in Phoenix. Numerous shootings of vehicles along I-10 over the past two weeks have investigators working around the clock to find a suspect in a spate of recent Phoenix freeway shootings that have rattled nerves and heightened fears of a possible serial shooter. (AP Photo/Matt York)
Arizona Department of Public Safety officers inspect a tractor trailer with a bullet hole in the passenger door shortly after it was shot near 67th Ave and I-10, Thursday, Sept. 10, 2015 in Phoenix. Numerous shootings of vehicles along I-10 over the past two weeks have investigators working around the clock to find a suspect in a spate of recent Phoenix freeway shootings that have rattled nerves and heightened fears of a possible serial shooter. (AP Photo/Matt York)
Map shows locations of 10 recent incidents on Phoenix-area freeways involving bullets or unspecified projectiles; 3c x 4 1/2 inches; 146 mm x 114 mm;
Leslie Allen Merritt Jr., makes his way to appear before a judge at the Maricopa County Sheriff's Office on Saturday, Sept. 19, 2015, in Phoenix. The landscaper is the suspect in a series of Phoenix freeway shootings and was arrested Friday after trying to sell a gun at a pawn shop. (Rob Schumacher/The Arizona Republic via AP, Pool)
An Arizona Department of Transportation sign gives a hotline number for information on the recent freeway shootings on Wednesday, Sept. 9, 2015 near Phoenix, Ariz. A bullet shattered the rear window of a truck on I-10 between 43rd Ave. and 35th Ave. in Phoenix earlier Wednesday. (AP Photo/Traci Carl)
This undated photo released by the Arizona Department of Public Safety shows a bullet hole in the windshield of a vehicle in Phoenix. Four vehicles traveling on the same freeway in the heart of Phoenix have been struck by gunshots in the last week, and the state police agency is appealing for information from the public as it conducts what it says is a robust investigation. (Arizona Department of Public Safety via AP)
This Saturday, Aug. 29, 2015 photo provided by The Arizona Department of Public Safety shows a SUV window shattered by a gun shot in Phoenix. Authorities say shots were fired at several vehicles on Interstate 10 in Phoenixover the weekend, with bullets striking three vehicles and injuring one person. (The Arizona Department of Public Safety via AP)
An Arizona Department of Transportation sign gives a hotline number for information on the recent freeway shootings as motorists pass under at the 202/I-10 intersection, Wednesday, Sept. 9, 2015 in Chandler, Ariz. Authorities are investigating nine shootings of vehicles over the past two weeks along I-10 in Phoenix. A truck's passenger window shattered on a Phoenix freeway Wednesday as Arizona authorities investigated a string of highway shootings that have rattled nerves and heightened fears of a possible serial shooter.(AP Photo/Matt York)
Danger on the freeway. Why @Arizona_DPS has doubled a reward to catch a shooter. Details on #GMAZ at 430am http://t.co/d54CEvr64W
INFO: @Arizona_DPS investigating NINE incidents in 10 days tied to freeway shootings. http://t.co/K0wDPE8mwD #abc15 http://t.co/EW9l7cNuFe
More than 100 calls coming in during the past 24 hours: http://t.co/rQGZaI69Nl
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The sergeant was not in uniform and was not injured, police said, adding that it could not immediately be determined if the vehicle was struck by gunfire or something else such as road debris.
Details of the second incident were not disclosed.
"It's just a matter of time, if this continues, that we have tragedy on our roadways," Milstead said at a news conference.
Authorities also announced on Tuesday that a reward fund has been increased to $20,000 from $5,000 for information leading to the arrest and conviction of those responsible.
Police said that they have not confirmed that all the incidents involved gunfire, saying only that some were hit by a projectile. Investigators also have not yet determined whether the incidents are connected.
On the first reported day of the shootings, a bus with only the driver on board was hit and a third vehicle was struck late that night, with the driver noticing a bullet hole after inspecting her vehicle the next day, officials said.
Milstead said the agency was using its gang task force, special weapons and tactics personal and undercover detectives for freeway surveillance, and was working with federal and local law enforcement.
He urged motorists to be on guard and report any suspicious activity, branding the recent incidents "domestic terrorism."
"If you drive that stretch of roadway, always be vigilant," he said. "Under these circumstances, be hyper vigilant."