Arizona authorities confirmed Thursday that 11 vehicles area have been shot with bullets or other projectiles in the last two weeks, nearly all of them on freeways in the Phoenix area.
The latest confirmed shooting involved a commercial truck whose driver reported a bullet hole in the cargo area Thursday morning. The driver had been making deliveries for hours, and wasn't sure when or where the truck was shot, Arizona Department of Public Safety spokesman Raul Garcia said.
Most of the vehicles hit since the gunfire began on Aug. 29 were traveling on Interstate 10, a main route through central and west Phoenix. No one has been seriously injured, but one bullet shattered a windshield, and broken glass cut a 13-year-old girl.
"Anytime that you have multiple shootings against American citizens on a highway, that's terrorism," Department of Public Safety Director Frank Milstead said. "It's just a matter of time before there is a tragedy."
Authorities also were studying a tractor-trailer rig at a location near I-10 and a car whose window was severely cracked Thursday, but those results weren't conclusive, Garcia said.
The shootings have rattled nerves and heightened fears among drivers that a possible serial shooter could hit them next. Electronic freeway billboards urge people to call a hotline with any tips. Some are commuting on city streets instead.
"I go through the streets when I go home," said Juan Campana, who works an appliance business in the area ofPhoenix where many of the shootings have happened, and watched helicopters overhead Wednesday after the 10th reported shooting.
It's unclear if the attacks are connected. Most were hit by bullets; some projectiles were harder to identify.
The FBI, the Bureau of Alcohol, Tobacco and Firearms and local police are helping. Authorities are conducting surveillance and deploying undercover detectives; a police SWAT team and gang task force also are assisting, and police have quadrupled a reward to $20,000 for help identifying a suspect.
But Graves said details of the surveillance effort and investigation will not be revealed.
"We're not going to give the nuts and bolts of our investigation," Graves said. Doing so "would help the bad guy."
The Phoenix attacks recall 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 before those criminals were captured.
In Michigan last year, a man was convicted of terrorism after shooting 23 vehicles on or near Interstate 96. Authorities in Ohio caught and sent to prison a man who took shots at several cars and houses over several months in 2003, killing one person.
But other shooters have eluded capture.
A series of apparently random roadside shootings in northern Colorado this year, which killed a cyclist and wounded a driver, raised alarm that a serial shooter might be trolling roads there. So far, the task force in Colorado and authorities in Arizona have found no links, said David Moore, a spokesman for the Larimer County Sheriff's Office.
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 the task force investigating the Michigan shooting spree in 2012.
"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.
AP writer Sadie Gurman in Denver contributed to this report.