Phoenix motorists taking Interstate 10 over the long Labor Day weekend will have more to worry about than just holiday traffic.

They face the unnerving prospect that a windshield, a side door, or the gas tank of their vehicle could be pierced by a random bullet fired by an unknown assailant.

That's because the Arizona Department of Public Safety is investigating a string of shootings that happened over a 48-hour period last weekend along I-10.

In all, four vehicles were hit by gunfire at different locations on the freeway, from 16th Street to as far west as 59th Avenue, according to the DPS.

A 13-year-old girl was injured in the first shooting at 11:03 a.m. Saturday, suffering a cut to her right ear as she rode in the front passenger seat of a sports-utility vehicle headed eastbound on the freeway.

The DPS says she was treated at the scene by firefighters and was not transported to a hospital.

The other shootings did not result in injuries. A tour bus headed west between 35th Avenue and 59th Avenue, empty save for the driver, was struck on its side at 11:09 a.m. the same day.

The third shooting happened about 10:15 p.m. Saturday when a female driver heard a loud bang. She did not check her vehicle until Sunday, according to the DPS, when she discovered bullet holes on the side of her car.

The fourth shooting occurred early Monday morning about 4:25 a.m. near 24th Street.

"A man driving a work truck heard a loud bang and thought that a rock struck the company truck," the DPS states.

Later, the driver inspected the vehicle and realized "the driver side front headlamp had damage consistent with a bullet strike."

In a recent interview with CNN, Colonel Frank Milstead, DPS director, explained that ballistic work still is under way at his agency's crime lab.

"We don't know that the rounds are related," he said. "But I will tell you after 30 years of law enforcement experience, there's very few coincidences of this magnitude. I think we'll find something being related."

The shootings call to mind the period from 2005 to 2006 when Phoenix was terrorized by three serial killers: Mark Goudeau, the "Baseline Killer," and Dale Hausner and Samuel Dieteman, known before capture as the "Serial Shooters."

Goudeau was a prolific rapist and murderer, believed to have killed nine people in all, as well as having kidnapped and raped several women in the Phoenix area before he was arrested September 2006.

Goudeau is on Arizona's death row for one of his slayings.

Hausner, an airport custodian and amateur boxing photographer, partnered with his roommate, Dieteman, in a crime spree of drive-by shootings where the pair would take aim at pedestrians, bicyclists, and animals.

They were apprehended August 2006. Hausner was later convicted of six murders and numerous related charges.

Sentenced to death, he cheated the executioner in 2013, committing suicide by swallowing an overdose of antidepressants.

Dieteman copped a plea and was sentenced to life without parole for two of the murders in 2009.

It was a creepy time in Phoenix, with people fearing to venture out at night because of the killings.

There also have been comparisons made in the local media between this current spate of shootings and those of DC snipers John Allen Muhammad and Lee Boyd Malvo, who killed 10 men and women during an October 2002 spree in the Virginia, Maryland, Washington, D.C., area (and several before the 2002 spree, including one victim in Arizona).

Muhammad was executed in 2009 for his crimes. Malvo is serving six life sentences.

At the time, one particularly eerie message from Muhammad and Malvo to the police involved the death card from a Tarot deck, inscribed with the words "Call me God."

But at this point, there is a very significant difference between the sniper sprees of the past and the shootings that have occurred recently along I-10.

No one has been murdered or seriously injured.

Let's hope it stays that way.

The PS encourages tipsters with information on the shootings to contact the agency via the DPS website, or by calling 1-877-2-SAVEAZ.

