SANFORD, Fla. -

A Sanford police car parked near the Trayvon Martin shooting scene was found Tuesday morning with several bullet holes in it, according to authorities.

The cruiser was parked across the street from the Retreat at Twin Lakes apartment complex in the parking lot of Bentley Elementary School.  Sanford police said the marked vehicle was shot around 4:30 a.m. by "unknown persons."

Police said no one was inside the cruiser, but gunfire shattered a passenger window and at least two bullets pierced the front windshield.  Witnesses said they heard six shots.

Police released pictures of the cop car Tuesday afternoon, showing the bullet holes in the windshield and passenger window, along with the left side fender of the car riddled with bullet holes.

Sanford police said the cruiser had been parked in front of the school as a visible deterrent because tour buses park during there day and night.

Police removed the cruiser from the scene, and no one was injured.  An investigation is ongoing. The cruiser has been brought to the processing center at the police department.

Sgt. David Morgenstern said police don't know if the shooting was in retaliation for not arresting Martin's gunman, George Zimmerman.

"I don't know what this is," Morgenstern said when asked what the shooting was in response to. "It's stupid and dangerous."

Morgenstern said he couldn't say if there were any cameras at the gates or outside of the school to capture who the shooter was. At this time, police don't know who is responsible for the shooting.

Martin, 17, was shot and killed in late February by neighborhood watchman Zimmerman, who claims he acted in self-defense.

Zimmerman, 28, has not been charged with a crime in Martin's death, a fact that has provoked demonstrations and calls that he be prosecuted for killing the teen.

On Monday, a group of students calling themselves the Dream Defenders marched to the Sanford police station. Six of the demonstrators wore hooded sweatshirts, as Martin did the night he was shot, as they blocked the department's main entrance; others linked arms, sang and chanted as they stood facing the building.

The demonstration closed the Police Department headquarters, and City Manager Norton Bonaparte Jr. and Acting Police Chief Darren Scott met with leaders of the student group and community leaders.

Although details of the Feb. 26 incident remain murky, what is known is that Martin, who was African-American, ventured out from his father's fiancee's home in Sanford to get a snack at a nearby convenience store. As he walked home with a bag of Skittles and an Arizona iced tea, he was shot and killed by Zimmerman, who is Hispanic, and who had called 911 to complain about a suspicious person in the neighborhood.

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