A man carrying a duffel bag walked into a rear door of the L.A. Fitness center in Collier this evening and opened fire, killing at least four people and wounding as many as 10.

Allegheny County police said they believe 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 of them -- all with multiple gunshot wounds who initially were listed in critical condition. Three later were upgraded to serious condition.

At St. Clair Hospital, spokesman Rich Seiber said three women were brought there with bullet wounds. One of them died of a chest wound at 8:54 p.m.

The other two, in their 20s and 30s, were in good and stable condition, one with a shoulder wound and the other with a leg wound.

Stacey Falk, 26, of Bridgeville, said she was in an aerobics room about 8:15 p.m with 30 to 40 other women when a short, clean-shaven man, dressed in workout clothes, walked in with a duffel bag. He stood in a corner near a racquetball court for a moment, put the bag down and turned off the lights in the room.

Then shots rang out, and people began falling to the floor. Ms. Falk said she briefly got a look at what appeared to be a short-barrel rifle.

"Girls were just ducking behind each other. All I could hear were gunshots and girls screaming, screaming. Girls were running into each other. We were using each other for protection.

"There was blood on the floor and casings all over the place. I didn't know it was real until I saw the casings on the floor."

At a press conference late tonight, Allegheny County Police Superintendent Charles Moffat said the gunman had two weapons.

"He walked right into the room as if he knew exactly where he was going," Superintendent Moffat said.

Detectives were working to identify the dead and the wounded.

"As you can imagine, at a fitness club, they don't have their IDs on them, people don't know who they are," he said.

Collier officers were called to the fitness center at 8:16 p.m., and the shooting had already ended. Police said the gunfire was contained to an aerobics room, where detectives recovered the two guns. Three bodies remained inside the room this morning. The fourth victim died en route to St. Clair Hospital.

Neither Superintendent Moffatt nor Collier Police Chief Thomas Devin could offer a motive for the shooting, saying the investigation was just beginning.

"I've never seen anything quite like this," the superintendent said.

Ashley Ogordowski, 23, of Muse, Washington County, said she was in the aerobics room when the man 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 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 has 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.

Ms. Dooley was on a treadmill, listening to her iPOD.

"I saw people flying off the treadmills, hitting the ground" for cover, Ms. 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 was playing racquetball said the scene inside 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 counted about eight shots.

Ms. Ross at UPMC Mercy said stress-management professionals from Western Psychiatric Institute and Clinic were dispatched to the center to counsel families of the victims.

A spokesman for the LA Fitness chain, Simon R. Barker, issued a statement from the company late tonight.

"Each of us in the LA Fitness family are shocked and saddened by the senseless acts of violence that took place. . . this evening. Our thoughts and prayers are with the victims, their families and friends. We are working with local law enforcement as they conduct their investigation."