Authorities in Arizona are investigating 11 confirmed shooting incidents within two weeks—most reported along the I-10— that include a confirmed gunshot early Thursday that left a hole in the side of a commercial tractor-truck.

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.

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.

In addition to the one confirmed shooting Thursday, authorities are investigating two additional possible shootings the same morning.

"Anytime that you have multiple shootings against American citizens on a highway, that's terrorism"

- Frank Milstead, director of Department of Public Safety

Milstead's agency brought in the FBI, the Bureau of Alcohol, Tobacco and Firearms and local police to assist in the investigation. Authorities have been 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.

Raul Garcia, a state trooper public information officer, told FoxNews.com that anyone who witnesses a shooting should call 911. He did not describe the firearm used and would not confirm media reports that there may be copycats.

"What we have is a very dangerous situation and somebody knows something," he said. "You need to let law enforcement know."

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.

FoxNews.com's Edmund DeMarche and The Associated Press contributed to this report