Four vehicles traveling on the same freeway in the heart of Phoenix have been struck by gunshots in the last week, and the state police agency is appealing for information from the public as it conducts what it says is a robust investigation.

The shootings confirmed by the Arizona Department of Public Safety occurred over a three-day span. The first occurred Saturday morning, followed by two more that evening. The fourth occurred Monday morning.

As police investigate and field calls from a jittery public, here is a look at what is known about the shootings and the probe:

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HOW DID THE SHOOTINGS PLAY OUT?

Four vehicles — a tour bus, work truck and two personal vehicles — were hit by at least one bullet each as they drove along the main freeway through the Phoenix metro area.

The shootings occurred at separate locations along 8 miles of Interstate 10. People in the vehicles heard loud noises, but some didn't realize their vehicle had been hit until later. A 13-year-old girl riding in an SUV was injured after suffering a cut to her ear from broken glass.

The bullets hit various parts of the vehicles, shattering a windshield, hitting a headlight and piercing the sides of two of them. The tour bus was empty except for the driver.

"You should be concerned," Col. Frank Milstead, the director of DPS, said earlier in the week. "We have some people or person shooting at moving vehicles on I-10. It has not ended in tragedy, but it could."

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WHAT'S KNOWN ABOUT THE INVESTIGATION?

The Department of Public Safety has not disclosed much information so far. Milstead said the vehicles apparently were chosen randomly.

No information has been released on a possible motive, including whether there's more than one shooter, what type of gun was used, or whether shots were believed to have been fired from a vehicle or locations along the freeway.

"We don't know what the intent is. It's not known at this point if it's a copycat crime," Milstead said.

Agency spokesman Bart Graves said investigators are contacting other states where similar shootings have occurred to see if there's a correlation.

"This group of detectives is working literally around the clock to try to get it solved," Graves said.

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HAVE INVESTIGATORS RECEIVED VALUABLE TIPS?

Milstead on Monday publicly asked for tips and information related to the shootings, resulting in approximately 100 calls to the agency, including an unspecified number of reports of possible hits to other vehicles. However, no additional hits have been confirmed, Graves said.

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ARE POLICE HOLDING BACK INFORMATION?

While DPS officials said the agency is still trying to nail down key circumstances of the case, Milstead said he didn't want to release preliminary information that later might prove incorrect or to give anything away that might "give this perpetrator a leg up on how to avoid detection."

Milstead also said he didn't want to put out information that might influence somebody to not contact authorities with information.

"It doesn't matter what you think you saw or what seemed odd," Milstead said. "If it causes you (to) pause and you're thinkin, 'I might have seen something,' please call us and let us be the ones to determine (whether) what you think you personally saw is valid or a concern."

Graves later said the agency suggests submitting tips through an online form for the Arizona Counter Terrorism Information Center at http://www.azactic.gov/Tips .