Clark County Sheriff Joseph Lombardo, who is investigating the Las Vegas shooting, has said he finds it hard to believe the man assembled his arsenal completely on his own.
It is not yet known why Stephen Paddock opened fire on an open air concert from the Mandalay Bay Hotel overlooking the venue, killing 58 people in the most deadly shooting in modern US history.
He took his own life before police stormed the room.
Mr Lombardo told reporters he found it hard to believe that the weapons, ammunition and explosives recovered by police in their investigation could have been assembled by Paddock completely on his own.
In a press conference today, Mr Lombardo said police had found more explosives in Paddock's car at the hotel, along with about 1600 rounds of ammunition.
"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," the sheriff said.
"You have to make an assumption that he had some help at some point," Lombardo said at a news briefing. Lombardo said the attack was the obvious outcome of meticulous planning.
He said Paddock had been gambling just hours before he began shooting, and had booked into the Ogden Hotel overlooking a different open air festival in Las Vegas a week earlier.
But he said Paddock's motivations and whether there were any possible accomplices remained a mystery. The FBI has said so far no link to terrorism had been found.
"We don't understand it yet," Sheriff Lombardo told reporters.
Mr Lombardo also gave a timeline of the police response during the attack:
- 10.05pm - First shots fired by the suspect. This was seen on closed-circuit television from the concert venue.
- 10.12pm - First two officers arrive on the 31st floor and announce gunfire is coming from directly above them.
- 10.15pm - Last shots are fired from the suspect per body-worn camera.
- 10.17pm - First two officers arrive on the 32nd floor.
- 10.18pm - A security officer tells Las Vegas Metropolitan Police Department officers he was shot and gives them exact location of suspect's room.
- 10.26-10.30pm - Eight additional officers arrive on the 32nd floor and begin to move systematically down the hallway, clearing every room and looking for any injured people. They move this way because they no longer hear the gunfire of an active shooter situation.
- 10.55pm - Eight officers arrive in stairwell at the opposite end of the hallway nearest to the suspect's room.
- 11.20pm - The first breach was set off and officers entered the room. They observed the suspect down on the ground and also saw a second door that could not be accessed from their position.
- 11.27pm - The second breach was set off, allowing officers to access the second room. Officers quickly realised there was no one else in the rooms and announced over the radio the suspect was down.
Mr Lombardo also said Paddock had fired 200 rounds of ammunition at the door and hallway after officers approached.
Police said he may have been planning to escape instead of shooting himself dead, but did not give further details.
Girlfriend 'I loved him and hoped for a quiet future together
Earlier, Paddock's girlfriend Marilou Danley has said she had no idea what her "kind, caring, quiet" partner was planning.
Ms Danley, who spoke to the FBI on Wednesday after travelling back voluntarily from a holiday in the Philippines, expressed shock at the "horrible unspeakable acts of violence" Paddock had committed.
Paddock "never said anything to me or took any action" which she understood as a warning of what was to come, she said in a statement read by her lawyer.
Ms Danley added: "I loved him and hoped for a quiet future together."
US authorities named Ms Danley a "person of interest" in their investigation and said they had made contact with her shortly after the shooting.
Ms Danley voluntarily flew back to Los Angeles from the Philippines on Tuesday night to speak to the FBI, just over two weeks after Paddock had surprised her with a "cheap ticket" to enable her to visit her family.
While there, he wired her US$100,000, explaining it was to buy a house.
"I was grateful, but honestly I was worried it was a way for him to break up with me," she said. "It never occurred to me in any way whatsoever that he was planning violence against anyone."
Her sisters earlier told Australian outlet 7News that Ms Danley "was sent away ... so that she will not be there to interfere with what he's planning".
Paddock checked into a suite in the Mandalay Bay Hotel on 28 September, reportedly using some of Ms Danley's identity documents.
President visits first responders
US President Donald Trump, who visited the city on Wednesday, said "America is truly a nation in mourning" in the wake of the mass killings.
Mr Trump praised the emergency services who battled to save as many as they could, despite the danger to themselves.
"When the worst of humanity strikes - and strike it did - the best of humanity responds," he said as he applauded injured officers.
"In the depths of horror, we will always find hope in the men and women who risk their lives for ours," he added.
Mr Trump said he was in the "company of heroes" after visiting the first responders.
"Words cannot describe the bravery that the whole world witnessed on Sunday night," he said. "Americans defied death and hatred with love and with courage."
First Lady Melania Trump joined the president to meet some of the victims and emergency responders on Wednesday.
Mr Trump told reporters at the University Medical Center in Las Vegas: "I have to tell you it makes you very proud to be an American when you see the job that they've done."
- Reuters / BBC