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THE sheriff leading the investigation into the Las Vegas shooting believes the gunman was not acting alone and most likely had an accomplice.
Speaking to reporters, Sheriff Joseph Lombardo said Stephen Paddock "had to have some help”.
Mr Lombardo said investigators did not have any other suspects at this stage, but they were "determined to find out if there was”.
"You look at the weapon obtaining, the different amounts of Tannerite (explosives) available, do you think this was all accomplished on his own?” he said.
"On face value, you've got to make the assumption he had to have some help at some point, and we want to ensure that that's the answer.”
Mr Lombardo's frank comments put into doubt explanation previously put forward by authorities that he was a "lone wolf”.
In explaining his theory, the sheriff mentioned Paddock's arsenal of 47 guns and a "plethora” of ammunition, plus his two Nevada residences.
He also made mention of his girlfriend, former Australian resident Marilou Danley, who has been named as a "person of interest” in the investigation.
"Maybe he's a super guy that was working this all out on his own, but it would be hard for me to believe that,” the sheriff said.
Federal Bureau of Investigation special agent in charge Aaron Rouse confirmed that Ms Danley had been interviewed after she flew into Los Angeles from the Philippines Tuesday overnight. She is not in police custody.
He also said that investigators had no evidence to indicate Paddock's attack was linked to international terrorism.
Earlier it was revealed police were one floor below Paddock just seven minutes after he began his massacre but it took another hour to storm his room.
Authorities have released a timeline of events from the start of the killer's mayhem to the time a SWAT team blasted their way into his 32nd floor suite at the Mandalay Bay Resort.
Paddock began firing thousands of rounds of bullets at 10.05pm local time (4.05pm AEDT), killing 58 people and injuring 489 others.
Sheriff Joseph Lombardo of Clark County said the first responding officers were one level below Paddock by 10.12pm and in the hallway near his room not long after.
He attempted to explain the "appearance of an undue delay” in a SWAT team entering at 11.20pm (5.20pm AEDT).
Sheriff Lombardo said a hotel security guard was shot when the killer fired through his hotel door.
When officers reached the hallway outside the room, they spotted a surveillance camera in place on a room service trolley outside Paddock's door.
They could also see inside through the bullet damage and spotted a high-powered firearm and "pulled back and waited for the approach of a full SWAT team”, Sheriff Lombardo said.
After being interviewed by FBI agents in Los Angeles on Wednesday, local time, Ms Danley released a statement through her lawyer, Matthew Lombard.
"I knew Stephen as a kind, caring, quiet man,” it read. "I loved him and hoped for a quiet future together. 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.
"It never occurred to me in any way whatsoever that he was planning violence against anyone.
"I am devastated by the deaths and injuries that have occurred and my prayers go out to the victims and their families and all those who have been hurt by these awful events.”