In their effort to find any hint of his motive, investigators were looking into whether he was with a prostitute days before the shooting, scrutinising cruises he took and trying to make sense of a cryptic note with numbers jotted on it found in his hotel room, a federal official said.
So far, examinations of Paddock's politics, finances, any possible radicalisation and his social behaviour, typical investigative avenues that have helped uncover the motive in past shootings, have turned up little.
The FBI announced that billboards would go up around the city asking anyone with information to phone 800-CALL-FBI.
"If you know something, say something," said Aaron Rouse, agent in charge of the Las Vegas FBI office. "We will not stop until we have the truth."
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Paddock, a reclusive 64-year-old high-stakes gambler, rained bullets on the crowd at a country music festival Sunday night from his 32nd-floor hotel suite, killing 58 and wounding hundreds before taking his own life.
Investigators believe Paddock hired a prostitute in the days leading up to the shooting and were interviewing other call girls for information, a US official briefed by federal law enforcement officials said. The official wasn't authorised to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.
The official also disclosed that Paddock took at least a dozen cruises abroad in the last few years, most of them with his girlfriend, Marilou Danley. At least one sailed to the Middle East.
"The lack of a social media footprint is likely intentional," said Erroll Southers, director of homegrown violent extremism studies at the University of Southern California. "We're so used to, in the first 24 to 48 hours, being able to review social media posts. If they don't leave us a note behind or a manifesto behind, and we're not seeing that, that's what's making this longer."
He requested an upper-floor room overlooking the festival, stockpiled 23 guns, a dozen of them modified to fire continuously like an automatic weapon, and set up cameras inside and outside his room to watch for approaching officers.
In a possible sign he was contemplating massacres at other sites, he also booked rooms overlooking the Lollapalooza festival in Chicago in August and the Life Is Beautiful show near the Vegas Strip in late September, according to authorities reconstructing his movements leading up to the deadliest mass shooting in modern US history.
Paddock bought 1,000 rounds of the .308-caliber and .223 -caliber tracer ammunition from a private buyer he met at a Phoenix gun show, a law enforcement official not authorised to comment on the investigation said on condition of anonymity. Tracer rounds illuminate their path so a gunman can home in on targets at night. But they can also give away the shooter's position.
Investigators are looking into Paddock's mental health and any medications he was on, McMahill said.
His girlfriend, Danley, told FBI agents Wednesday that she had not noticed any changes in his mental state or indications he could become violent, the federal official said.
Paddock sent Danley on a trip to her native Philippines before the attack, and she was unaware of his plans and devastated when she learned of the carnage while overseas, she said in a statement.