So enormous was the tragedy that, surely, it must have had some logical explanation.
But 12 months removed, the violence and fear that George Sodini unleashed inside the LA Fitness Center in Collier -- randomly killing three women and wounding nine others before shooting himself -- was based on nothing more than his hatred of the opposite sex.
"There was nothing we saw that could have prevented this," Lt. Bill Palmer of the Allegheny County Police homicide squad said of the subsequent investigation.
"No one could have had or would have had any suspicions there was that kind of danger. We didn't see anything that would have alerted anyone that he was capable of this.
"There was nothing LA Fitness could have done better. There was nothing anyone could have done. He hadn't even talked to his family for months. He was deranged and disturbed."
Armed with four guns, Mr. Sodini, 48, of Scott, a member of the health club in the Great Southern Shopping Center, apparently had no specific targets in mind when he walked into an aerobics class crowded with women shortly after 8 p.m. one year ago today.
Investigators believe he picked that class for no other reason than it was exclusively for women. In writings that turned up after his death, Mr. Sodini was discovered to be a self-loathing misfit, rejected by women for years.
Dressed in black, he turned off the lights and used two of the weapons to fire 36 shots, killing Heidi Overmier, 46, of Carnegie, Elizabeth Gannon, 49, of Green Tree, and Jody Billingsley, 37, of Mt. Lebanon. He then fired a 37th round from a third gun into his head.
It lasted about a minute but had been in the planning for months.
The next day, authorities discovered on the Internet Mr. Sodini's often rambling diary dating from Nov. 5, 2008, to the night before the shootings. The postings indicated he had planned the attacks on women and his own suicide at an unnamed gym for up to nine months, backing out of one attempt in January 2009 and rehearsing his murderous routine in the days preceding the attack.
County police computer-crimes investigators reviewed the diary's history to see how long it had been on the Internet and whether anyone had read it and not done anything. They discovered that although Mr. Sodini had written the diary during the timespan indicated, he had not posted it on the Internet until the night before the shootings.
Bridgeville police Officer Roger Itzel was among the first to arrive on the chaotic scene.
"I didn't think it would bother me as much as it did," said Officer Itzel, who had to watch over the aerobics classroom until all the evidence -- including the victims' bodies -- had been gathered. He still recalls the lingering stench of gunpowder, the ringing cell phones left behind by their owners in their rush to safety, and a blood trail leading out the door. "I can still see them, lying there. It's still as vivid as the day it happened."
For Officer Itzel and fellow Bridgeville Officer James Lancia Jr., who have more than 34 years of law enforcement experience between them, the senselessness of the shooting made it one of the more troubling things they've seen.
"These people were just normal everyday people, trying to better themselves, and here they're victims," Officer Lancia said.
Both officers were among those who gathered in the Collier police station shortly after the shooting, where counselors told them not to feel guilty -- they had done the best they could. They were offered time off, but neither took it. They tried to put it behind them and move on.
But at a recent training session at Chartiers Valley Middle School, a slideshow of deadly shootings included scenes from LA Fitness, which "just brought it all back" for Officer Itzel.
"I had to look away," he said.
After the shootings, he stopped working out at the Collier LA Fitness and for the first time started carrying his gun while off-duty. He went to the victims' viewings and paid his respects.
Around Christmas, Ms. Gannon's boyfriend, Michael Interthal, brought him a card at the station. "Thank you for your support in our time of need," it said. "My life was forever changed when I lost Betsy. The only thing that helps is knowing how much people care."
"That meant a lot to me," Officer Itzel said.
Tonight, Ms. Overmier's family will gather at the First United Methodist Church of Bridgeville to dedicate a memorial in her honor at the church where she was so active. Family gatherings have been bittersweet this year, said one of her sisters, Connie Moneck.
"It's like your impulse is to call Heidi. You've got to realize she's not here," she said. "It's such a benchmark, this first year. It's really been a transition period for our family."
Ms. Overmier's son, Ian, now 16, moved to South Carolina to live with his father. Friends and community members here raised money for his education.
Ms. Overmier's siblings were left with the emotional task or sorting through her belongings in the days after the shooting. She would have turned 47 last Sunday.
Her sister's full life -- and startling death -- are rarely far from Ms. Moneck's mind.
"Sometimes I try to relive that all and wonder just what happened to her. Did she suffer? I wonder if she knew what happened to her, I wonder if she missed us. I wonder if she ever had time to think of any of that," she said. "I know I can't put myself in that situation to where I would ever know for sure what happened inside that gym. But it's scary. You rethink that all the time."
One week after the tragedy, lifelong Collier resident Doreen Ducsay organized a candlelight vigil in the center's parking lot attended by more than 500 people. At the time, she said it was a way for a community that was "sick" about the tragedy to honor the dead, support the wounded and comfort each other. The moving vigil mourned the loss of life, the terror and the suffering while acknowledging the community's resiliency of spirit and the need to persevere.
"Did they heal?" Ms. Ducsay wondered aloud about her neighbors last week. "I don't know if they're ever going to heal. I think they know they have to move on and try to make it better."
That's happened at LA Fitness, where men and women continue to lift, swim and work out in the large facility. LA Fitness officials in Collier and at corporate headquarters in Irvine, Calif., would not comment for this story.
After the shootings, the health club closed for nearly three weeks, reopening on Aug. 22. Members who returned that morning offered sympathy for the victims but said the reopening was an apt metaphor of how life must move on.
In fact, an aerobics class held that morning in the classroom where the shootings occurred -- its walls brightly repainted and flooring replaced -- was crowded.
Perhaps the lasting effect, Ms. Ducsay said, is that Collier no longer has the naivete that "it can't happen here." And, she said, that has made residents "more aware of their surroundings ... Things that were taken for granted before are not taken for granted anymore."
And in that, she said, there is a sadness -- a community's longtime innocence destroyed in a minute by a self-hating misogynist, his monstrous act as chillingly arbitrary as it was meticulously planned.