PHOENIX (AP) -- 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.
The state Department of Public Safety has not yet confirmed whether the glass was shot out. Nobody was hurt, agency spokesman Bart Graves said.
Authorities were already investigating nine shootings of vehicles over the past two weeks. Four cars were hit last week along the city's main freeway. One bullet shattered a windshield and the broken glass cut a 13-year-old girl. The other rounds hit a headlight and the sides of vehicles.
Then on Tuesday, police announced that they were investigating five more shootings, including one that shattered the window of an off-duty police sergeant's vehicle as he drove to work. On Wednesday, investigators raced to a gas station after the driver of a white truck pulled off Interstate 10 with a shattered window.
Department of Public Safety Director Frank Milstead called the incidents "domestic terrorism crimes."
"Anytime that you have multiple shootings against American citizens on a highway, that's terrorism," Milstead said. "They're trying to frighten or kill somebody."
His agency brought in the FBI, the Bureau of Alcohol, Tobacco and Firearms and local police to help. Authorities were conducting surveillance and deploying undercover detectives and members of the SWAT team and a gang task force.
"Ten days, nine incidents," Milstead said at a news conference. "This is a real and continuing threat to Arizona motorists."
Juan Campana works at an appliance business near where many of the shootings occurred. He was surprised to look up and see the helicopters over the scene of Wednesday's incident.
Campana said he's not taking the freeway anymore out of concern about the shootings.
"I go through the streets when I go home," he said.
Police have been asking for the public's help in identifying a suspect, including putting messages on freeway billboards urging people to report suspicious activity. They quadrupled the reward Tuesday to $20,000.
Graves said the agency would not discuss the surveillance or other aspects of its investigation, including whether any bullets had been recovered, whether the shooter or shooters fired from vehicles or alongside freeways and whether more than one weapon was involved. Police do not know if all nine of the shootings are connected or whether a copycat might be at work.
"We're not going to give the nuts and bolts of our investigation," Graves said, adding that doing so "would help the bad guy."
Investigators don't know a possible motive for the shootings, Graves said.
Milstead said drivers are fortunate that no one has been killed or seriously hurt, but if the incidents continue, "it's just a matter of time before there is a tragedy."
The shootings began Aug. 29, when two vehicles were struck in a half-hour span on Interstate 10 between 19th and 59th avenues. A third vehicle was hit on the same freeway near 16th Street later that day.
A fourth vehicle was shot Aug. 31 in the same area.
On Tuesday, a passenger window on the Phoenix police sergeant's personal car shattered while he was driving to work before dawn on I-10 between 35th and 43rd avenues, according to DPS officials. The officer was not injured. Another shooting happened nearby a minute later.
In a statement, Arizona Gov. Doug Ducey said "the safety of Arizonans is our No. 1 priority, and we are committed to apprehending those responsible for these crimes."
The shootings brought back memories of other random highway and roadside shootings that have occurred around the country over the past decade, most notably the sniper attacks that terrorized the nation's capital more than a decade ago.
In other cases, a man was convicted last year of terrorism charges after opening fire on a busy Michigan highway because he believed the drivers were part of a government conspiracy against him. An Ohio man took shots at several cars and houses over several months in 2003, killing one, before being caught and sent to prison.
Law enforcement officials who helped crack the cases in Ohio and Michigan described the difficult task of nabbing a suspect in highway shooting investigations.
The efforts require a large number of officers who are ready to flood an area immediately after shots are fired, said Lt. Ron Moore, who commanded a Michigan task force that investigated the 2012 spree in which 23 vehicles were shot on or near Interstate 96.
"You have to bring all the resources you can to bear on the problem - and that's exactly what we did," said Moore, an officer in Wixom, Michigan.
Copyright 2015 The Associated Press modified.