WILKINSBURG, Pa. — A shooting at a backyard party that killed five and wounded three — many of them related — in this suburb of Pittsburgh was planned and calculated, a prosecutor said Thursday.

The prosecutor, District Attorney Stephen A. Zappala Jr. of Allegheny County, said at a news conference that one of the gunmen, who was in the alley, fired a handgun to drive the victims in the backyard toward the house’s porch, where a second gunman was able to pick them off with an assault-style rifle. All of the dead were shot in the head, he said.

No suspects have been identified, officials said, and investigators are searching for a motive for the shooting, which occurred shortly before 11 p.m. Wednesday. The gunmen apparently ran away.

All of the dead at the scene — three women and one man — were found on the porch, Lt. Andrew Schurman of the Allegheny County police said. A fourth woman was taken to the University of Pittsburgh Medical Center Mercy, where she died.