A gunman who walked into an exercise class, turned off the lights and shot three women dead before killing himself - may have been seeking revenge on an ex-girlfriend.
The ex-lover was among the group inside the gym near Pittsburgh.
"He walked right into the room where the shootings occurred as if he knew exactly where he was going," police chief Charles Moffatt said. "I think he went in with the idea of doing what he did."
The man wounded nine others before shooting himself.
The violence rocked Collier, a commuter community of about 5,300, 10 miles from Pittsburgh.
Joann Gazzam, a member of the weekly "Latin impact" dance aerobics class, saw the gunman walk to the back of the room near some weights, set down a bag and fumble with it for a few minutes before coming up with what appeared to be two guns and opening fire, according to her sister, Debi Wozniak.
Ms Gazzam told her that the instructor was among those who appeared to have been shot, and that the gunman had killed himself.
"She told me, 'Debi, I seen everything. Oh, my God, I seen everything. I seen him pull out the guns,"' said Ms Wozniak.
The clean-shaven gunman walked in the room wearing workout gear, turned off the lights and, at first nobody knew what was happening, said Stacey Falk, 26, who was in the class.
Police chief Moffatt said the gunman went into the health club planning to shoot several people - firing "multiple" weapons "indiscriminately," - and didn't say anything before unleashing a burst of bullets.
The gunman was a member of the health club and had identification, but that police needed to check fingerprints and speak to his family to confirm his identity.
Police found at least two guns at the scene and that a note was found in the shooter's bag.