WILLIAM ANDERSON'S grandparents had only one question last night as they sat in the Hospital of the University of Pennsylvania.

"Why?"

Why would a group of armed thugs attack their grandson, a delivery driver for a Wynnefield pizzeria who police say has never had a run-in with the law?

The short answer, according to police, is greed: The three men called Pete's Pizza, on 54th Street near Arlington, at 1 a.m. yesterday looking to rob one of the restaurant's drivers.

Anderson, 30, answered that call, arriving at an apartment complex on 63rd Street near Jefferson in Overbrook to find a group of at least three attackers waiting for him.

"They definitely had this planned," said Lt. John Walker of the Southwest Detective Division. "They had a car parked nearby, and they knew they could make a quick hit."

Anderson tried to fight off his attackers and was shot three times - once each in the stomach and chest, with the third shot striking both of his legs as it entered and exited his body.

He was taken to HUP after the attack, where he remained in critical but stable condition last night after undergoing surgery.

Walker said it's not yet clear what was taken in the robbery, noting that Anderson's cellphone was found at the scene.

Employees yesterday at Pete's Pizza declined to comment out of respect for Anderson's family, saying they weren't sure if they had been notified of the shooting.

But they had, as the somber scene at HUP showed.

As medics cared for Anderson, who slipped in and out of consciousness, his grandparents, who didn't wish to be identified, said little.

They said they knew their grandson had worked for the pizza shop for a few years, and that this was the first time he had run into any sort of trouble on the job.

"He's a good person," his grandmother said. "We don't know why this happened."

Walker said police are reviewing surveillance footage from cameras near the apartment complex. Investigators were hoping to speak with Anderson late last night, if his condition improved.

Tipsters can contact police at 215-686-3183 or 215-686-TIPS (8477).

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