It was "almost like a terror attack," Homicide Capt. Jack Ryan said Monday.

Two shooters — likely teenagers — "indiscriminately" opened fire on a group of teens Friday night in West Philadelphia, killing 17-year-old Sandrea Williams and wounding two teenage boys, Ryan said.

Shortly before 10:30 p.m., the gunmen crept up a driveway on the 300 block of North Simpson Street and fired 23 shots in approximately seven seconds at a group of at least six teens hanging out near Simpson and Carlton Streets, Ryan said.

Williams, who had just driven to West Philly with a 17-year-old friend to visit friends and her mother, who lives on Simpson, was an apparent bystander. She was pronounced dead shortly afterward at Penn Presbyterian Medical Center. Her 17-year-old friend and another boy, 15, were each shot in a leg and hospitalized in stable condition.

Ryan declined to say who the shooters may have been targeting except to say it was likely not Williams.

Police believe the shooting was motivated by "some manner of petty neighborhood dispute," said Ryan, who declined to elaborate. Police were still investigating Monday and working on collecting surveillance video, he said.

Fired cartridge casings were found from two guns — a .45-caliber and a 9mm. "It's a miracle that there weren't any more casualties," Ryan said.

In an interview Saturday, Williams' mother, Nadia Syblis, 34, said she had been sitting on her front steps talking to an older brother when she heard the gunshots. She ran inside, came back out after she saw someone lying on the 6400 block of Carlton, then realized it was her daughter.

Doctors at Penn Presbyterian told her that the girl was hit by a bullet that entered her back and exited her neck, the mother said.

The teen had lived with her mother and two sisters, ages 10 and 18, on Simpson, but had moved in February to East Mount Airy to live with an aunt and help care for her three children, family members said. Williams was a junior at Camelot Academy in North Philadelphia after spending her freshman and sophomore years at Overbrook High School, her mother said.

A $20,000 reward is offered for information leading to an arrest and conviction. Tipsters can call the Citizens' Crime Commission at 215-546-TIPS or homicide detectives at 215-686-3334.