PITTSBURGH - A man carrying a duffel bag walked into a rear door of the L.A. Fitness center in the suburb of Collier last night and opened fire, killing four or five people and wounding as many as 11 more.

Allegheny County police said they believed the gunman was among the dead.

The wounded were transported to UPMC Mercy, Allegheny General, and St. Clair hospitals.

UPMC Mercy spokeswoman Linda Ross said her facility received five victims - all women with multiple gunshot wounds who initially were listed in critical condition.

Ashley Ogordowski, 23, of Muse, Washington County, said she was in an exercise class when the man came in and started shooting.

"I was crouched to the side with a girl who was not my friend . . . shoot . . . shoot . . . shoot," she said, sobbing.

She said she was told that several people had been wounded, including a friend of hers.

"All I want to do now is find out what happened to my friend."

The scene outside the fitness center was chaotic as dozens of ambulances, police cars, and other emergency vehicles converged on the area.

Tony Williams said his daughter, Melinda, 22, told him that she was in the aerobics area "when he came in with a bag and started shooting." He said he couldn't say what kind of weapon the gunman used.

His daughter, he said, was shot in the knee and fell to the floor. A friend helped her to safety.

"She's going to be OK," the distraught father said.

Many of those who got out of the gym safely credited the fitness center staff with calmly steering people toward the exits.

After the shooting, scores of those people milled about the parking lot outside the fitness center, which had been taped off and was inaccessible to those who left personal property inside.

Lauren Dooley, 26, was inside the center when the shooting broke out on the first floor.

Dooley was on a treadmill, listening to her iPod.

"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."

She said she heard 12 to 15 shots, possibly coming from an aerobics room.

A man who had been playing racquetball said the scene was total chaos.

"It was panic," said Perry Calabro, 53, of Bridgeville. "Then we realized it was gunfire. Once the shooting started, everybody started running."

He said he counted about eight shots.