A man responsible for a deadly shooting rampage at a fitness club in suburban Pittsburgh was a 48-year-old loner who seethed online about his anger over women regretting his advances.

George Sodini, despite his best efforts to look good, hadn't had a girlfriend since 1984 and hadn't slept with a woman in 19 years.

"Women just don't like me. There are 30 million desirable women in the US (my estimate) and I cannot find one. Not one of them finds me attractive," the computer programmer wrote in an online diary post.

Sodini had worked as a systems analyst in the accounting and finance department of a Pittsburgh law firm since 1999.

A website in Sodini's name details his life of rejection by women, struggles with alcohol and bad relationships with family members.

Sodini listed his status as "Never married," and posted his date of death as Aug. 4, 2009.

Sodini entered an aerobics class at an LA Fitness centre in Collier Township, about 13 kilometres southwest of downtown Pittsburgh, on Tuesday night.

He placed a duffel bag on the ground, took out at least two guns and began shooting, police said.

Three women were killed and nine others were injured.

According to police, Sodini may have fired 52 shots before killing himself.

Allegheny County police Superintendent Charles Moffatt said the gunman fired "multiple" weapons "indiscriminately," and gave no warning before he began firing upon his victims.

"He walked right into the room where the shootings occurred as if he knew exactly where he was going," Moffatt said. "I think he went in with the idea of doing what he did.

"He just had a lot of hatred in him and (was) hell-bent on committing this act, and no one was going to stop him."

Sodini was a member of the gym and was there twice on Tuesday before the attack, police said.

Police said he had no criminal record and legally bought the weapons used.

A chilling plan laid out online

The website, which is registered to Sodini, contains what appears to be information about the nine months leading up to the shooting.

"The biggest problem of all is not having relationships or friends, but not being able to achieve and acquire what I desire in those or many other areas," reads an entry dated Sunday. "Everything stays the same regardless of the effert I put in. If I had control over my life then I would be happier. But for about the past 30 years, I have not."

It appears the attack has been in the works since November, with the gunman seeming to back out of an attempt on Jan. 6.

"It is 8:45PM: I chickened out!" he wrote. "I brought the loaded guns, everything. Hell!"

Police say they are trying to determine if anyone read the posts before the rampage.

"If anyone knew of it, they would have a moral and ethical obligation and legal obligation to bring it forward," Moffatt said.

Terrifying witness accounts

Joann Gazzam told her sister, Debbie Wozniak, that the gunman walked to the back of the room at the fitness club, put down his duffle bag, pulled out two guns and started shooting.

"She told me, 'Debi, I seen everything. Oh, my God, I seen everything. I seen him pull out the guns,'" said Wozniak, who normally attends the same class.

Twenty-six-year-old Stacey Falk of nearby Bridgeville, Pa., said the gunman turned off the lights in the aerobics class before he started shooting.

"All of us girls were just ducking behind each other and it was just, you know, I was behind a girl, one of the girls in front to get hit, and when he was in the opposite corner shooting, I booked it," Falk told WPXI-TV.

Loretta Moss, 44, who lives southwest of the fitness centre in McDonald, Pa., said she was on a treadmill when she heard a popping noise.

"I didn't pay attention, and the next minute, people were screaming," said Moss.

More pops followed.

"There was like a whole spray of them. I'd say about 15 altogether, and then people started screaming and yelling and started running out of the building," Moss said. "We laid down, and then after the last set of ... gunshots, we got up, and someone said run."

Moss said she saw two women who were shot during the rampage -- one who was shot in the leg and the other in the shoulder.

The woman who was shot in the leg was screaming in pain.

"She was screaming: 'It's burning, just please call the ambulance," Moss said.

By early Wednesday, the remaining nine victims were in three separate hospitals. Moffatt said two women remained critical, two others were in surgery, three others were hospitalized and another two had been treated and released from hospital.

The victims were later identified as Heidi Overmier, 46, of Carnegie, Pa., 49-year-old Elizabeth Gannon of Pittsburgh and 38-year-old Jody Billingsley of Mount Lebanon, Pa.

The police superintendent confirmed that police found four guns guns and a note inside the shooter's duffle bag -- but he would not say if the gunman was the author of the note.

The fitness centre released a statement about the shootings: "Each of us in the LA Fitness family are shocked and saddened by the senseless acts that took place."

One of Sodini's neighbours told police that he was "so anti-social we really didn't learn anything personal about him."

Sodini's family issued a short statement: "Our hearts and prayers are with the victims and their families and we pray for the full recovery of the survivors."

With files from The Associated Press