When the first bullets cracked through the air over a country music festival on the Las Vegas Strip last year, many concertgoers thought they were hearing fireworks. It took a moment to realize it was gunfire, a barrage of bullets that seemed endless, they recalled afterward.

“I remember my husband saying, ‘Get down — get down,’” a 33-year-old woman from California who came to Las Vegas to attend the festival would later tell police. She was then hit in the upper left thigh: “It was within a few seconds of being on the floor I got shot instantly.”

She was one of 22,000 people at the Route 91 Harvest festival, a three-day country music concert, when a gunman perched in a 32nd-floor suite at the nearby Mandalay Bay Resort and Casino opened fire. He killed 58 people — the youngest was 20, the oldest was 67 — before taking his own life as police closed in. Another 869 people were injured, nearly half of them by gunfire or shrapnel, on top of countless others carrying the mental anguish that comes from living through the harrowing carnage and the confusion that came with the attack.

The Las Vegas massacre is the deadliest mass shooting in modern American history, a tragedy without clear comparison to the other recent shooting rampages that have cut down people in schools and movie theaters, churches and offices. Each rampage is its own nightmare, spurring an outpouring of sorrow, anger and grief affecting students in Parkland, Fla., churchgoers in Sutherland Springs, Tex., club patrons in Orlando and a church group in Charleston, S.C., among others.

In Las Vegas, gunman Stephen Paddock fired for more than 10 minutes, police said, shooting more than 1,000 rounds at the concert venue, nearby airport fuel tanks and in his hotel hallway before turning a gun on himself.

“It seemed to last forever and ever and ever and ever,” one man who was shot in the upper shoulder at the concert told detectives.

The nightmare of Oct. 1 gave the world stories of unimaginable horror and heroism, tales of people confronted with mortal, terrifying danger and risking life and limb to save one another. First responders and civilians alike rushed into the fray, tearing off shirts to use as tourniquets, taking the wounded into their cars, comforting strangers, throwing their bodies on top of others to shield them.

“It was incredible,” a woman who was shot in the chest later told police in an interview at the hospital. Men she didn’t know put pressure on her wound, dragged her to safety, put her on a board and carried her out, getting her into a white pickup truck, she said.

Among the enduring, painful legacies of that night has been what remains frustratingly out of reach. After mass killings, authorities often seek to provide some answers about the “why” — what could have motivated the attacker, fueled their rage, spurred them to kill unsuspecting bystanders. A year after the Las Vegas massacre, no answers have been forthcoming. The Las Vegas police, in their final investigative report on the shooting, said they were unable to determine what motivated the attack.

“This report was authored to provide the reader with more information about who, what, when, and where,” authorities wrote in the document, which was released in August. “Regretfully, this report will not be able to address the why.”

The report passed with relatively little notice in a world that shifted its attention from the Las Vegas massacre to the gunfire at a church in Texas and a high school in South Florida and a school in Texas, but it provided a grim reality.

In many cases, mass violence has been preceded by warning signs visible to the people in the attackers’ lives. In a study released this year, the FBI examined dozens of active shooters who opened fire between 2000 and 2013 and found that on average, they had displayed several “concerning behaviors.” These warning signs, which become glaring red flags in hindsight, have a way of focusing the public pain and attention after a tragedy, as questions arise over what could or should have been done to prevent the violence and what, if anything, could be done to avert such horrors in the future.

Sometimes the warning signs are glaring and ignored. After the Parkland, Fla., massacre in February, authorities acknowledged failing to act on repeated warnings that the confessed attacker posed a threat. After Las Vegas, though, authorities said what they found failed to add up to a clear answer.

Investigators said the gunman was described by people who knew him as “a narcissist [who] only cared about himself.” Relatives said he had mental-health issues, while his doctor said he might have had bipolar disorder and that he accepted prescriptions for anxiety medication but declined anti-depressants. His wealth had declined considerably before the shooting; officials found that he had paid off his gambling debts before the attack and, with one of his last checks, paid the Internal Revenue Service more than $13,000. Authorities said he spent significant time preparing for the attack as he stockpiled weapons and ammunition. They also said he sought to “undermine” the investigation and left no note or manifesto.

In response to questions about the shooting and the attacker’s motive, an FBI spokeswoman would only say that the bureau’s Behavioral Analysis Unit plans to release a report by the end of the year.

For some victims, the mystery was part of the pain that is left behind. Megan Greene, who survived the Las Vegas attack and struggled with anxiety afterward, said knowing the motivation could have aided with the healing process.

“It would help us understand and wrap our brains around the situation,” Greene said in an interview earlier this year. “It would be easier for me to understand more of why this happened to me. … It’s just a puzzle to all of us.”

Greene said she spent months searching for new information on the case but gave up on that around Christmas.

“I didn’t want to feel miserable anymore,” she said.

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