A gunman walked into a Bridgeville, Pa., health club tonight, turned out the lights and opened fire, witnesses of the shocking attack said.
The shooting left four people dead -- including the shooter, a law enforcement source told ABC News radio. Up to 15 others have been reported injured.
At a press conference Tuesday night, Allegheny County police Superintendent Charles Moffatt said that three women and one man were among the dead.
Collier Township Police Chief Tom Devin told reporters that he believed the shooter committed suicide at the scene but could not yet be sure.
Two weapons have been found at the scene.
Witnesses told ABC News affiliate WTAE in Pittsburgh that a man turned off lights and shot people in a Latin dance class in the aerobics center of the L.A. Fitness gym at approximately 8 p.m.
A woman in the class at the time of the shooting, who identified herself as Nicole, told WTAE that the class had started about 10 minutes before the gunfire.
She said about 30 women were in the class when "a middle-aged white male walked into the class. He had a big gym bag."
"He looked out of place in a class full of women," according to the witness, who said the man put his bag down, turned the lights off and opened fire.
Moffat said that the man did not say a word upon entering the gym and then walking into the dance studio.
"He walked right into the room where the shootings occurred as if he knew exactly where he was going," he said.
The floor was littered with shell casings and blood, according to surviving witnesses.
A local hospital is treating five women who were shot multiple times, all of whom were in critical condition, reports WTAE.
Area hospitals are treating the other victims, including two in fair condition.
LA Fitness gym staff told WTAE that it was a busy night health club at the time of the shooting, with about 100 people inside. Witnesses said people inside the gym took off running amid the gunshots.
Lauren Dooley, 26, told the Pittsburgh Post-Gazette, that he had been running on a treadmill, listening to her iPod when the chaos broke out.
"I saw people flying off the treadmills, hitting the ground" for cover, Dooley said. "We crawled through the fire escape, and I sprinted out the back."
"I turned around there was one girl shot in the thigh and one girl shot in the back," Branson Holly, a witness inside the gym at the time of the shooting told WTAE. "I'm still shaken because I was in spinning class and it could have been my class."
Many heard the sounds of gunfire, others actually saw shots being fired in the darkened room.
"The lights were off and I could see flashes in the dark," said Jared Frederick, a witness.
Debi Wozniak told the Associated Press that she is normally a regular at the weekly Latin impact dance exercise class. She says she missed the class Tuesday but her sister, Joann Gazzam, had attended.
Gazzam told Wozniak that a man came in through the glass double doors, walked to the back of the room and set down the bag. She said he began looking through his bag and pulled out and began shooting.
"She told me, 'Debi, I seen everything, oh my God, I seen everything. I seen him pull out the guns,"' Wozniak said.