LAS VEGAS (WTVD) -- As investigators continue to hunt for a motive and other clues in the mass shooting in Las Vegas, here are the latest details:
A U.S. official said Friday that investigators believe the Las Vegas shooter may have hired a prostitute in the days before the shooting, and they are interviewing other call girls as they look for clues into his motive.
The official said that prostitutes are among the hundreds of leads they are pursuing as part of their investigation into Stephen Paddock.
The official, who was briefed by federal law enforcement officials, wasn't authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.
The official says a note containing a series of numbers was found on a nightstand in Paddock's room at the Mandalay Bay hotel after the shooting.
The official also says that Paddock had taken at least a dozen cruises out of the U.S. in the last few years, most with his girlfriend Marilou Danley. The official said at least one of the cruises was to the Middle East.
AUTHORITIES PUTTING UP BILLBOARDS
Authorities are planning to put up billboards in Las Vegas to seek more tips as they investigate the deadliest shooting rampage in modern U.S. history.
Undersheriff Kevin McMahill also revealed at a news conference Friday afternoon that police are confident there was not another shooter in Paddock's room.
McMahill also said that authorities don't have any information that anyone else used Paddock's room key.
He says authorities are interested in Paddock's medical history and are looking into that.
NO CLEAR MOTIVE
Authorities say they still don't have a clear motive for the Las Vegas shooting rampage.
Undersheriff Kevin C. McMahill provided an update on the investigation Friday. He says authorities have looked at gunman Paddock's personal life, political affiliation, economic situation and any potential radicalization.
He says authorities are aware the Islamic State group has claimed responsibility for the attack, but so far there is no evidence that it had a role.
He says authorities will continue to investigate those areas as well as look into leads and tips that come in.
SHOOTER LACKED 'SOCIAL MEDIA FOOTPRINT'
Paddock's anti-social personality will only hamper investigators as they try to figure to piece together what led to the shooting.
"It's extremely difficult," said Erroll Southers, the director of homegrown violent extremism studies at the University of Southern California.
"The lack of a social media footprint is likely intentional," Southers said. "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.
"What's really puzzling is that we've seen him with similar kinds of activity - booking rooms in other places - so you have to ask yourself the reason he picked Las Vegas and not somewhere else."
Paddock fired indiscriminately Sunday from his upper-level room at the Mandalay Bay hotel casino at people attending a country music festival below, killing dozens and injuring nearly 500 people. The 64-year-old Paddock killed himself as authorities closed in.
Because so few people knew Paddock well, investigators will likely have an even harder time sorting through his background to try to uncover any possible leads, Southers said.
"You don't have any cases of leakage - no one to say who's he mad at, what his motive is," Southers said. "The key to this case right now is the girlfriend."
"The reason you want to engage in a terror attack is you want to bring attention to an extremist ideology, you want publicity," he said. "You want people to be afraid of what you believe what you do."
SHOOTER MENTIONED RELATIONSHIP TROUBLES
Scott Armstrong, a Reno, Nevada, car dealer, said Paddock confided in him about relationship troubles when the would-be gunman stopped in looking to buy a car about two months ago.
"Somehow or another we ended up talking about bad relationships, and he confided that he was depressed and his life was miserable," Armstrong said. "It just struck me as really odd that somebody would say that."
Paddock, unshaven and dressed casually but not disheveled, didn't elaborate on his relationship troubles, and Armstrong didn't pry. He didn't smile, and he "wasn't very pleasant to talk to," he said.
"I could tell he was really down or something," Armstrong said. "I just told him, I said, 'Hey I've been in some bad relationships myself. It'll get better. Tomorrow will be a better day than today.' "
Armstrong said he's talked with FBI agents about his recollection.
Others who have interacted with Paddock have described him as a quiet and confident man who did not engage in deep conversations. Armstrong said he's baffled by why he was so open with a stranger, but "my job is to put people with ease and try to help them buy a car."
How is he so certain Paddock is the downtrodden man who visited him?
"When's the last time somebody told you their life was miserable? It sticks with you," he said
BOUGHT 1,000 ROUNDS OF TRACER AMMO
A law enforcement official says Paddock bought 1,000 rounds of tracer ammunition a month ago from a private seller he met at a Phoenix gun show.
The official says investigators searching the hotel room Paddock used as a sniper's perch found tracer rounds and a document with the name of the Mesa, Arizona, man who sold him the ammunition.
The official is involved in the shooting investigation and spoke anonymously because the official was not authorized to disclose case information.
Tracer bullets contain a pyrotechnic charge that illuminates the path of fired bullets so shooters can see whether their aim is correct.
The official says Paddock met the man in Phoenix on Sept. 9 and 10 and that the sale took place at the man's Mesa home.
The tracer ammunition that Paddock bought were .308-caliber and .223-caliber rounds.
The official did not know whether Paddock used tracer rounds during the attack. The official declined to identify the seller.
Information from The Associated Press