One year after the deadliest mass shooting in US history, experts are no closer to determining why the attacker, 64-year-old gambler Stephen Paddock, laid down a hail of bullets at the Route 91 Harvest Festival, killing 58 concertgoers and injuring over 400 who suffered gunshot wounds.

On Monday, the Las Vegas strip will "go dark" at around the same time as Paddock opened fire on the crowd, in commemoration of the dead.

Marquees along the Strip will go dark at 10:01 p.m. in a tribute that will last several minutes. The iconic signs also went dark on Oct 8, 2017, one week following the horrific tragedy at the Route 91 Harvest Festival. -LA Times

Experts remain puzzled as to Paddock's motive after the Las Vegas Metropolitan Police Department spent 10 months investigating, interviewing his relatives, girlfriend, ex-wife, doctor and casino hosts, reports the Wall Street Journal, which notes that "several hypotheses on the Las Vegas gunman’s possible psychopathy and desire for infamy have begun to emerge, but they are tentative and based on limited evidence—a troubling outcome for people whose job it is to look for clues that could help prevent such a deadly incident in the future."

"People are bewildered by the case—there’s a bewilderment, and there’s a horror," said UC San Diego forensic psychologist, J. Reid Meloy. "The most troubling cases are those without an answer."

Sheriff Joseph Lombardo, head of the Vegas police, said upon the release of the final report in August that Paddock’s gambling losses may have been a factor; his bank accounts dwindled from $2.1 million to $530,000 in the two years before the attack. But the sheriff said investigators weren’t able to “definitively answer the why.” -WSJ

"We wish we knew more about it," said psychologist and threat-assessment expert John Nicoletti. "With all the missing data, what everybody says, it’s just speculation."

FBI criminal profilers, meanwhile, have been working on their own Paddock report that is expected to be released soon. The agency's top official in Vegas said in a summer radio interview that the report may not deliver "a definitive why."

"It’s a puzzling case and a challenging case," said retired FBI profiler Mary Ellen O’Toole. "In a lot of ways, he is an outlier."

From the start, Paddock defied much of what professionals in this grim field have come to expect. Typically, mass shooters are younger men who nurse real or perceived grievances, according to a recent FBI study of 63 such attackers. Four out of five displayed some concerning behavior before an attack, including telling others on social media or in person of their violent intent. Paddock was more clandestine. Video-surveillance footage shows him calmly gambling, eating and bringing more than 20 pieces of luggage up to his 32nd floor suite in the Mandalay Bay Resort & Casino in the days before the attack. The luggage contained an arsenal of semiautomatic rifles, and he turned his room into a sniper’s nest to rain bullets down on a defenseless crowd at the Route 91 Harvest country music festival on Oct. 1, 2017. -WSJ

In the ensuing months, investigators were unable to find any animus which might explain Paddock's actions. That has left some experts to conclude that his motive was infamy.

"Some people kill for notoriety and infamy, and that’s what he did," said Dr. Russell Palarea, president of the threat-assessment firm Operational Psychology Services.

Meloy, the UCSD psychologist, thinks that Paddock's motive is tied to his father - who was a bank robber and con man who was on the FBI's Ten Most Wanted Fugitives list in 1969, and determined to be a sociopath.

"I began to think about psychopathy, Meloy told the WSJ. "when I was struck by the history of the dad and the fact that his biology was rooted in a father who had a diagnosis as a psychopath."

Meloy says that speculation by Paddock's brother in interviews that "he had done everything in the world he wanted to do and was bored with everything" supports his hypothesis, along with the fact that several other people described Paddock as emotionally detached. Meloy is not alone in this theory.

The coldblooded and grandiose assault on a crowd of people Paddock had never met also bore characteristics of psychopathy, not of someone having a mental breakdown, said Dr. O’Toole, the retired FBI profiler. “It was a complete lack of empathy for the trauma and damage done to strangers,” she said. -WSJ

That said, the theory does have holes - including Paddock's lack of a violent past, impulsiveness and lying. Paddock called his mother before the attack to make sure she was safe ahead of Hurricane Irma. He also shared his investments and wealth with family and friends, such as his girlfriend, Marilou Danley.

Paddock also took prescription medication to control anxiety, typically not a trait seen in those with no conscience.

"In order to say that Stephen Paddock was a psychopath, you would have to do a posthumous assessment with case materials, you’d have to do interviews, you’d have to go back over years of behavior," said O’Toole.

One law enforcement official close to the investigation has his own theory: "My opinion is he was pissed over getting his butt kicked gambling, or he wanted to follow in his father’s shoes."