He intended to escape after the attack, but he instead turned the gun on himself as authorities closed in on his room on the 32nd-floor of the Mandalay Bay Resort and Casino.
"He was doing everything possible to see how he could escape at this point," Lombardo said.
Before killing himself, Paddock, 64, had set up cameras inside and outside his hotel suite, and in the peep hole.
Police also found 50 pounds of explosives and 1,600 rounds of ammunition in his car in the Mandalay Bay hotel parking lot, according to Lombardo. There was also a note in his hotel room, but it was not a suicide note, he said, without providing details on what it said.
Nearly 500 concertgoers were wounded either by the barrage of bullets or in the stampede when Paddock opened fire.
Did killer have an accomplice?
Lombardo painted a picture of a man who may have had some help in planning the deadliest mass shooting in modern US history.
More than 100 investigators have spent the past three days combing through details of his life, looking for answers.
"What we know is Paddock is a man who spent decades acquiring guns and ammo, and living a secret life," Lombardo said. "Anything that would indicate this individual trigger point, which would cause him to inflict this harm, we're not there yet."
Investigators said something may have happened to Paddock between October last year and September this year that compelled him to purchase more weapons and ultimately lead to the massacre. Paddock bought 33 firearms, mostly rifles, during that 13-month period, authorities said.
"Do you think this was all accomplished on his own? You've got to make the assumption he had to have some help at some point," Lombardo said.
He highlighted the killer's many weapons and explosives in his car. Investigators say evidence shows Paddock meticulously planned the shooting.
"It's troublesome this individual was able to move this amount of gear into a hotel room unassisted," Lombardo said. " It's troublesome for the amount of stuff he had at both residences unassisted."
The arsenal of weapons in his hotel suite included bump-fire stocks -- legal devices that enable a shooter to fire bullets rapidly, similar to an automatic rifle.
The first call reporting shots fired came at 10:08 p.m. Sunday, and the gunfire didn't stop until 10:19 p.m., Clark County Undersheriff Kevin McMahill said. The shooting continued off and on for nine to 11 minutes, he said.
Before targeting the Route 91 Harvest music festival, Paddock rented a room at a condominium complex overlooking the Life is Beautiful music festival, another downtown event held between September 22 to September 25.
The sheriff said the room was not rented by the Ogden, but by a private owner.
"Was he doing pre-surveillance? We don't know yet, this is all conjecture at this point," Lombardo said.
Few answers, more questions
Days after the attack, investigators remain baffled about Paddock's motives even after talking to his girlfriend.
In a statement through an attorney, Marilou Danley said she didn't know he planned to carry out a mass shooting.
She said he bought her a ticket to the Philippines about two weeks ago, then wired her moneyso she could buy herself a house there. At the time, she was worried he wastrying to break up with her.
Paddock wired $100,000 to the Philippines, a law enforcement source said, but officials haven't determined when the money transfer took place or who was the recipient. The FBI is working with Philippine authorities to get more details.
"It never occurred to me in any way whatsoever that he was planning violence against anyone," Danley said in the statement.
Danley lived with Paddock in Mesquite, Nevada, and was out of the country during the shooting.
She returned to the United States on Tuesday night from the Philippines and spoke to the FBI and the Las Vegas police, according to her attorney, Matt Lombard.
"I will cooperate fully with their investigation," she said in the statement. "Anything I can do to help ease suffering and help in any way, I will do."
Motive still a mystery
Almost three days after the deadliest mass shooting in modern American history, investigators appear no closer to answering the question: Why?
Why did the 64-year-old fire with an arsenal of weapons for nine to 11 minutes? What pushed him to target a crowd of 22,000 in the heart of Las Vegas?
FBI Deputy Director Andrew McCabe told CNBC that the lack of a clear motive was a "surprise" in this mass shooting.
"This one is somewhat different than many of the ones we've dealt with in the past, because we don't have any immediately accessible thumbprints that would indicate the shooter's ideology or motivation, or really what compelled him to get there," McCabe said.
The FBI is going through Paddock's communications, financial records, associates and video surveillance to try to piece together the puzzle of his motive.
Paddock is twice divorced, liked to gamble, and previously worked at the US Post Office and the IRS. He had no significant criminal history and was until then unknown to police.
FBI special agent Aaron Rouse said they have not found any evidence to indicate terrorism, but the investigation is ongoing.