An off-duty Philadelphia police officer allegedly shot and killed a man and wounded two other people in South Philadelphia on Sunday night after what witnesses said was an argument over a playful water battle.

The encounter happened around 6:30 on the 1800 block of Hoffman Street, according to police spokesman Lt. Frank Vanore.

When it was over, one man - identified by neighbors as Howard Williams, 22 - was taken to the Hospital of the University of Pennsylvania, where he was pronounced dead. A 26-year-old woman was taken to HUP with a gunshot wound to a leg. An 18-year-old man was grazed by a bullet and treated at HUP, Vanore said.

Vanore said the bullets were fired by the officer, whom he would not identify.

Residents of the street said the shootings followed a water battle involving children and adults. An altercation broke out between two women when one of them got angry at having been sprayed.

There was apparently bad blood between the women, and perhaps between the officer and Williams as well, according to accounts. Neighbors said one of the women was a friend or girlfriend of the off-duty officer. Williams tried to break up the fight, they said, but it was unclear whether that was what prompted the officer to draw his gun.

Eunice Simpkins, who identified herself as the street assistant block captain, said she was returning from the store when she heard people shout, "Howard! . . . He's not moving, he's not moving."

Simpkins called him "my Howie," and said he helped her and others in the area.

"It was senseless," Simpkins said. "I don't know why this man did this."

Homicide and Internal Affairs investigators were investigating Sunday night, and neighbors said most witnesses had been taken to Police Headquarters for questioning.

A family friend, who would only identify herself as Cece, said Williams was "a joy to be around." He loved basketball and stayed out of trouble, she said.

"Everybody loved him on this street," she said. "He put a smile on your face."

Contact staff writer Jeff Shields at 215-854-4565 or jshields@phillynews.com.