PHOENIX >> Authorities in Arizona were investigating two more possible shootings of vehicles in the Phoenix area, where electronic billboards urged freeway motorists to provide tips on a spate of shootings that have residents on edge.

The shootings have rattled nerves and heightened fears of a possible serial shooter, and some motorists have started avoiding using the freeways, instead taking city streets. No one has been seriously hurt in the shootings, although one bullet shattered a windshield and the broken glass cut a 13-year-old girl.

The state Department of Public Safety said a passenger car’s window was damaged when hit by an unspecified projectile on Interstate 10 Thursday morning. News video showed an intact but severely cracked rear window with a visible indention.

In another incident, the DPS said it was investigating a delayed report of a commercial truck being hit by a bullet. Investigators were near I-10 examining what appeared to be a small hole in the truck’s cargo area.

DPS spokesman Bart Graves said the truck had been making deliveries for hours before the damage was noticed and it’s unclear where or when it might have been hit.

Graves said it had yet to be determined whether either incident involved a shooting. He said investigators were unable to verify a separate report of a gunshot along I-10.

There have been at least 10 other confirmed shootings in the last two weeks, though it’s unclear if they all are connected. Most were on Interstate 10 — one of the main highways through the city — and seven of them involved bullets, with three others involving unspecified projectiles, DPS said.

Department of Public Safety Director Frank Milstead has 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.” He did not elaborate.

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.

Juan Campana works at an appliance business near where many of the shootings occurred. He was surprised to look up and see helicopters on Wednesday after the 10th reported shooting.

Campana said he’s not taking the freeway anymore.

“I go through the streets when I go home,” he said.

Police have been asking for the public’s help in identifying a suspect and quadrupled the reward Tuesday to $20,000.

Graves said officials would not discuss the surveillance or other aspects of its investigation.

“We’re not going to give the nuts and bolts of our investigation,” Graves said, adding that doing so “would help the bad guy.”

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 Phoenix shootings have brought back memories of other random highway and roadside shootings in recent years, most notably the sniper attacks that terrorized the nation’s capital more than a decade ago.

A series of apparently random roadside shootings in northern Colorado earlier this year raised alarm that a serial shooter might be trolling areas roads.

A member of the task force investigating the northern Colorado shootings that left a cyclist dead and a driver injured called authorities in Arizona to see if there were any similarities, said David Moore, a spokesman for the Larimer County Sheriff’s Office. Investigators found no links, he said.

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 person, before being caught and sent to prison.

Making an arrest in such cases requires 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.

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AP writer Sadie Gurman in Denver contributed to this report.