The gunman who carried out the worst mass shooting in recent US history had booked rooms at a Chicago hotel overlooking the Lollapalooza music festival in August, the website TMZ reported on Thursday.
Stephen Paddock booked two rooms at the Blackstone Hotel in downtown Chicago, overlooking an expansive park where the popular music festival took place over four days in August, TMZ said.
Although the website said Paddock never showed up for his booking, the report suggested a potential scenario similar to that which took place at a country music festival in Las Vegas on Sunday night.
Paddock rained bullets down on festivalgoers from a nearby hotel building, killing 58 and injuring hundreds more.
"We are aware of the media reports and have been in communication with our federal partners," Chicago police spokesman Anthony Guglielmi said.
The hotel management company did not immediately respond to requests for comment.
Hundreds of thousands attended the Lollapalooza festival at Chicago's Grant Park, including Malia Obama, former US president Barack Obama's daughter.
The Blackstone is a historic luxury hotel where heads of state, including former president Jimmy Carter, and artists such as Nat King Cole and Rudolph Valentino have stayed.
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Authorities believe Paddock had stockpiled weapons and ammunition over decades, and meticulously planned the Sunday night's attack.
But what led the 64-year-old to unleash the carnage remains largely a mystery.
"What we know is that Stephen Paddock is a man who spent decades acquiring weapons and ammo and living a secret life, much of which will never be fully understood," Clark County Sheriff Joseph Lombardo said on Wednesday night.
Mr Lombardo said he found it hard to believe that the arsenal of weapons, ammunition and explosives recovered by police in their investigation could have been assembled by Paddock completely on his own.
"You have to make an assumption that he had some help at some point," Mr Lombardo added.
Some 489 people were also injured when Paddock strafed an outdoor concert with gunfire on Sunday night from his 32nd-floor suite of the Mandalay Bay hotel on the Las Vegas Strip. He then took his own life.
Police recovered nearly 50 firearms from three locations they searched, nearly half of them from the hotel suite. Officials said 12 of the rifles there were fitted with bump stocks, allowing the guns to be fired almost as though they were automatic weapons.
Like other recent mass shootings, the incident stirred the debate in Washington over regulating firearm ownership, which is protected by the Second Amendment of the US Constitution.
Republicans, who currently control the White House and both chambers of Congress, have fought off Democratic calls for stricter background checks or federal limits on magazine size following past mass shootings.
But congressional Republicans said they would be willing to investigate the bump stocks that allow legal semi-automatic rifles to behave similarly to fully automatic weapons, which are largely illegal in the US.
Investigators were examining the possibility that Paddock's purchase of more than 30 guns in October 2016 may have been precipitated by some event in his life, Mr Lombardo said.
The FBI said on Wednesday there remained no evidence indicating that the shooting spree was an act of terrorism.
Paddock's girlfriend, Marilou Danley, was questioned by the FBI on Wednesday and said in a statement she had been unaware of his plans.
"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," Ms Danley, 62, said.
Ms Danley, who returned to the US late Tuesday from a family visit to the Philippines, is regarded by investigators as a "person of interest". Her lawyer, Matt Lombard, said his client was co-operating fully with authorities.
An FBI official in Las Vegas, meanwhile, said no one has been taken into custody.
An Australian citizen of Filipino heritage, Ms Danley said she flew back to the US voluntarily "because I know that the FBI and Las Vegas police department wanted to talk to me, and I wanted to talk to them".
Ms Danley shared Paddock's home at a retirement community in Mesquite, Nevada, north-east of Las Vegas, before travelling to the Philippines in mid-September.
Investigators questioned her about Paddock's weapons purchases, a US$100,000 (Dh367,317) wire transfer to a Philippine bank that appeared to be intended for her, and whether she saw any changes in his behaviour before she left the US.
Ms Danley said Paddock had bought her an airline ticket to visit her family and wired her money to purchase property there, leading her to worry he might be planning to break up with her.
Paddock's brother Eric said the $100,000 transfer was evidence that "Steve took care of the people he loved", and that he probably wanted to protect Ms Danley by sending her overseas before the attack.
Discerning Paddock's motive has proven especially baffling as he had no criminal record, no known history of mental illness and no outward signs of social disaffection, political discontent or extremist ideology, police said.