Las Vegas Sherriff Joseph Lombardo detailed the carnage carried out by Stephen Paddock, 64, from the 32nd-floor hotel room at the Mandalay Bay Resort and Casino.
The mass shooting killed 58 people and wounded nearly 500 others in a 10-minute barrage of gunfire.
At 10.05pm local time on Sunday, Paddock opened fire on the crowd at the nearby Route 91 Harvest Festival.
Just seven minutes later at 10.12pm, two officers arrived on the 31st floor of the Mandalay Bay hotel and said the gunfire was coming from above them.
Paddock then fired his last shots at 10.15pm as officers got onto the 32nd floor.
A security guard told officers he was shot and gave the location of the shooter's hotel room at 10.18pm.
More than 200 rounds were fired from the room into the hallway.
Between 10.26pm and 10.30pm, eight more police officers reached the 32nd floor of the hotel, and started clearing each room and searching for wounded.
Almost an hour later at 11.20 pm, officers used explosives to breach the door of Paddock's room, which was a suite. They found him on the ground and discovered a locked bedroom door.
Officers breached the second door at 11.27pm and determined no one else was present in the suite.
Paddock 'a kind, caring, quiet man'
Paddock's girlfriend Marilou Danley was out of the country when Paddock rained bullets down on concert-goers on the Las Vegas Strip on Sunday night.
She said she knew nothing about his diabolical plan.
“I knew Stephen Paddock as a kind, caring, quiet man,” Ms Danley said in a statement read out by her lawyer.
"I loved him and hoped for a quiet future together with him.
“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.”
Her brother, Reynaldo Bustos, trold ABC News he spoke to his sister following the shooting.
"I called her up immediately and she said, 'Relax, we shouldn't worry about it. I'll fix it. Do not panic. I have a clean conscience,'" Mr Bustos told ABC News in the Philippines.
Ms Danley returned to the United States from the Philippines on Tuesday night, according to US press reports, and was met by agents of the Federal Bureau of Investigation at Los Angeles airport.
“I was grateful, but honestly, I was worried, that first, the unexpected trip home, and then the money, was a way of breaking up with me,” she said in her statement.
“It never occurred to me in any way whatsoever that he was planning violence against anyone.”
Police have not described Ms Danley as a suspect but Las Vegas Sheriff Joe Lombardo said Tuesday she was a "person of interest" and law enforcement hoped she could shed some light on the investigation.
She is not under arrest or currently facing any charges.
No motive has emerged yet for the deadly shooting spree by Paddock, a wealthy, 64-year-old retired accountant who liked to gamble for high stakes and owned a comfortable home at a golf course retirement community near Las Vegas.
It was through his gambling that Paddock reportedly met Ms Danley, when she was working as a hostess at a casino in Reno, Nevada, several years ago.
'He loved her'
"They were adorable - big man, tiny woman. He loved her. He doted on her," Eric Paddock, Paddock's brother, told The Washington Post.
Paddock and Ms Danley shared a home in the town of Mesquite, 130 km northeast of Las Vegas, where the authorities found a number of guns while searching the property.
Neighbours have said the couple mostly kept to themselves and were frequently away for weeks at a time on gambling trips.
According to Australian authorities, the Filipino-born Ms Danley has Australian citizenship and has family there.
President Donald Trump talks with Las Vegas Mayor Carolyn Goodman and Clark County Sheriff Joseph Lombardo after arriving in Las Vegas. Source: AAP
Interviewed with her face blurred so she could remain anonymous, one of Ms Danley's sisters told Australia's 7 News she believed Paddock had deliberately sent her away.
"I know that she don't know anything as well like us. She was sent away. She was away so that she will be not there to interfere with what he's planning," she said.
According to ABC News, Ms Danley left the United States for the Philippines on September 15 about two weeks before Paddock checked into his room at the Mandalay Bay hotel and began the final phase of carrying out an attack that the authorities said was meticulously planned.
Other reports have said she left the country on September 25.
Nick Suarez, a spokesman for the Philippines' National Bureau of Investigation, said the NBI was looking into reports Ms Danley arrived in the Philippines last month, and that Paddock had sent $100,000 to her via a wire transfer.
Air Force One carrying President Trump taxis on the runway past Mandalay Bay on Wednesday, October 4, 2017, in Las Vegas. Source: AAP
According to The Washington Post, Ms Danley married a man named Geary Danley in Las Vegas in 1990 and they divorced in 2015.
The New York Times said both Geary Danley and Marilou Danley had children from prior relationships and Ms Danley has a daughter who lives in California.
"She’s probably one of the most happy, outgoing, full-of-life people I’ve ever known," Dionne Waltrip, Marilou Danley's former stepdaughter, told the Times.
"Everyone who has ever met her likes her."
Did the gunman have an accomplice?
Sherriff Lombardo said the scale of the preparations for the attack raise questions about Paddock potentially having had an accomplice, but none has been identified.
He told reporters he found it hard to believe 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," Sherriff Lombardo said at a news briefing.
Sherriff Lombardo said the attack was the obvious outcome of meticulous planning.
"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.
He also updated the number of people wounded in the attack, putting the latest figure at 489.
As the victims' harrowing stories emerged in the media, America once again grappled with calls for reforms to its permissive firearm control laws.
Mr Trump largely stuck to a talking point repeated by dozens upon dozens of Republican officials: "Well, we're not going to talk about that today. We won't talk about that."
US officials have reacted cautiously to a claim by the Islamic State jihadist group that the shooter had carried out Sunday night's massacre on its behalf.
Authorities said Paddock, who had no criminal record, smashed windows in his hotel room shortly after 10 pm on Sunday and rained fire on a crowd of some 22,000 attending the country music concert below.
When a SWAT team stormed the room where Paddock had been staying since September 28, they found he had killed himself.
Authorities have seized 47 firearms from three locations.
So far, investigators have found nothing to explain Paddock's actions, but were continuing to hunt and trace every possible clue about a gunman they described as a "psychopath."
