Those seeking to know the motive of Las Vegas gunman Stephen Paddock have had little more to chase than hints and shadows.
Paddock led such a low-key, private life that no one seemed to know him well, and those who did had no sense he was capable of the deadliest mass shooting in modern U.S. history.
Other mass killers have left behind a trail of plain-sight clues that help investigators quickly understand what drove them to violence. But Paddock, 64, had nearly no close friends, social media presence or other clear connections to the world.
Even the No. 2 official in the FBI said Wednesday he was surprised investigators have not uncovered more about why a man with no obvious criminal record would cause so much bloodshed.
"There's all kinds of things that surprise us in each one of these events. That's the one in this one, and we are not there yet," FBI Deputy Director Andrew McCabe said. "We have a lot of work to do."
Working with what little they know, investigators have zeroed in on Paddock's weapon-buying binge a year before he gunned down 58 people Sunday at a country music festival from a 32nd-floor room at the Mandalay Bay casino resort before killing himself. At least four Canadians are among the dead and six others were wounded.
Many of Paddock's weapons were fitted with so-called bump stocks — attachments that allow semi-automatic weapons to fire as rapidly as fully automatic weapons, which are subject to much stricter regulation in the United States.
In a rare concession to gun control advocates, the National Rifle Association (NRA) on Thursday said such attachments need tougher regulation.
The powerful gun lobby group called on federal authorities to "to immediately review whether these devices comply with federal law."
"The NRA believes that devices designed to allow semi-automatic rifles to function like fully automatic rifles should be subject to additional regulations," it said.
Investigators wonder if Paddock had some sort of mental break that drove him to start making plans for mass murder.
They also know, though not why, he rented an apartment in a Las Vegas highrise over another music festival the weekend before the massacre.
They know he was a major gambler and are looking at related records, though even in public casinos he played the private game of video poker.
They know he had a plan to survive the shooting and try to escape, though would not say how.
"This individual and this attack didn't leave the sort of immediately accessible thumbprints that you find on some mass casualty attacks," McCabe said. "Putting aside the somewhat dubious claims of responsibility that we see in each one of these instances, we look for actual indicators of affiliation, of motive, of intent, and so far we're not there. We don't have those sort of indicators."
Clark County Sheriff Joseph Lombardo said at a news conference Wednesday night that Paddock had a stockpile of weapons, explosive materials and a meticulous plan. Lombardo said those factors have led him to believe the shooter might have had some help — though he cautioned that investigators don't yet know if that's the case or who it might be.
Lombardo said investigators have been compiling a profile of a man he called "disturbed and dangerous."
He said Paddock spent decades acquiring weapons and ammunition, and "living a secret life, much of which will never be fully understood."
The sheriff, who said Paddock meticulously planned the deadly attack, appealed for any information anyone might have about the shooter
"Right now, we're trying to prove his intent, or understand his intent, and the history associated with this — and whether or not he has any accomplices.
"There's people that know this individual. There's people that could help us understand this individual."
Some who thought they knew him intimately could provide no help.
"He never said anything to me or took any action that I was aware of that I understood in any way to be a warning that something horrible like this was going to happen," Marilou Danley, 62, said in a statement read by her lawyer outside FBI headquarters in Los Angeles.
When asked Wednesday night whether there were specific individuals, other than Danley, who may be a person of interest, Lombardo said investigators are trying to determine if there were people who helped Paddock in any way.
"It's important not to close this case until we run down everything," the sheriff said, citing the volume of weapons and explosive materials Paddock had gathered.
Maybe Paddock worked everything out on his own, Lombardo said, but "it'd be hard for me to believe that."
Danley said she was initially pleased when Paddock wired her money in the Philippines to buy a house for her family — an apparent reference to the $100,000 US transfer made shortly before the massacre — but she later feared it was a way to break up with her.
A senior U.S. homeland security official, speaking to Reuters on condition of anonymity, said investigators believe the money was meant as a form of life insurance payment for Danley.
She said she loved Paddock as a "kind, caring, quiet man" and hoped they would have a future together. She said she was devastated by the carnage and would co-operate with authorities as they struggle to get inside Paddock's mind.
The previous weekend, Paddock had rented a highrise condo in a building that overlooked the Life is Beautiful alternative music festival featuring Chance the Rapper, Muse, Lorde and Blink-182, said Lombardo, who offered no other details about what led Paddock there.
On Sept. 28, the high-stakes gambler and real estate investor checked into Mandalay Bay and specifically requested an upper-floor room with a view of the Route 91 Harvest music festival, according to a person who has seen hotel records turned over to investigators.
Paddock wasn't able to move into the room until Saturday, said the person, who was not authorized to speak publicly and disclosed the information to the AP on condition of anonymity.
The room, which goes for $590 a night, was free for Paddock because he was a good customer who wagered tens of thousands of dollars each time he visited the casino, the person said.
Authorities are looking for hints in those details of the kind of life he lived, and the kind of victims and venue he targeted, said David Gomez, a former FBI national security and criminal profiler.
"We may never know to 100 per cent certainty," he said. "But they will find out."
With files from CBC News and Reuters