Las Vegas authorities have revised the timeline of the mass shooting, saying Stephen Paddock shot a hotel security guard around the time he began firing on the crowd, not six minutes earlier.

Speaking at a Friday news conference, Clark County Sheriff Joe Lombardo didn't offer any new information about what motivated Paddock to open fire on a country music festival from the 32nd floor of the Mandalay Bay Resort and Casino, leaving 58 people dead.

He said the number of wounded and injured is now 546, with 45 people still hospitalized, some of them in critical condition.

An autopsy was performed and the medical examiner's visual inspection found no abnormalities on Paddock's brain, Lombardo said. The brain has been shipped to another facility for "microscopic evaluation," he said.

The new timeline means police no longer think there was a six-minute gap between the shooting of the guard and the barrage on the concert crowd.

Lombardo said the sequence of events has changed as more information has been obtained. "This is a very dynamic event," he said, adding that more new facts may be reported.

Initially, police said security guard Jesus Campos approached Paddock's room as the October 1 shooting was underway, diverting the gunman's attention. Paddock then shot Campos through the door and quit firing at concertgoers at the Route 91 Harvest Festival from his hotel room on the 32nd floor, the timeline said.

Lombardo's timeline from Monday -- that Campos was shot at 9:59 p.m. and the shooting on the crowd started six minutes later -- raised questions about why police didn't make it to Paddock's room sooner to break down the door.

On Friday, Lombardo said Campos arrived near Paddock's suite at 9:59 p.m., but was not shot then. Police Sgt. Joshua Bitsko told CNN that Paddock had screwed shut the stairwell door to the hallway near his room. The security guard went to an upper floor and came back down to the 32nd floor by another door, he sheriff said.

Paddock shot Campos through the door about 10:05 p.m. and began firing on the crowd around then, Lombardo said.

Lombardo's latest timeline essentially agrees with one put forth Thursday by MGM Resorts International, owner of Mandalay Bay, which disputed the times earlier provided by police. MGM said Paddock was shooting at concertgoers "at the same time as, or within 40 seconds after," Campos first reported shots.

Why the confusion about what happened at 9:59 p.m.?

Lombardo said the time came from "human entry" in a security log.

"I still stand by the time of 9:59," Lombardo said Friday. "It wasn't inaccurate when I provided it to you. The circumstances associated with it is inaccurate."

MGM said that time "was derived from a Mandalay Bay report manually created after the fact without the benefit of information we now have. We are now confident that the time stated in this report is not accurate."

Police were with armed Mandalay Bay security officers in the building when Campos first reported over the radio that shots were fired, the MGM statement said. The officers and the armed security personnel immediately responded to the 32nd floor, MGM said.

Lombardo confirmed Paddock fired at fuel tanks at the nearby airport but said there was a low probability aviation fuel could be ignited by gunfire. He also said it appears Paddock fired on police officers as they arrived at the scene.

The lack of answers, especially about the timeline, seems all the more curious when, it would appear, many moments in the shooter's dayslong preparation -- and the actual assault -- were captured by hotel video surveillance or by cameras the gunman himself installed in his suite and hallway outside.

Federal and local agencies continue collecting evidence, delving into the gunman's life and interviewing those close to him.

Here's what we still don't know about the deadliest mass shooting in modern US history.

Why did Paddock fire for only 10 minutes?

Police said an open door to a room near Paddock's suite had set off an alarm, prompting Campos to respond to the floor. Once there, Campos heard a drilling sound from Paddock's room, and he was shot in the right upper thigh.

Paddock unleashed bullets on concertgoers for about 10 minutes. The police didn't get to Paddock's suite until 10:17 p.m. They didn't breach the room until 11:20 p.m., after a SWAT team arrived with explosive charges, Lombardo said.

It's unclear why Paddock stopped shooting into the festival crowd after 10 minutes.

When SWAT officers breached Paddock's door, they discovered the gunman dead, apparently of a self-inflicted gunshot wound.

Lombardo said Campos, upon being shot, notified security, and his action helped pinpoint the suspect's location.

How could he meticulously plan this kind of attack without anyone noticing?

Over several days, Paddock took guns and ammunition to his hotel suite. He had 23 weapons in his room. In his car were more than 50 pounds of explosives and 1,600 rounds of ammunition, police said. Twelve of the firearms had bump stocks -- legal accessories for rifles that enable them to be fired more rapidly.

Authorities also found a note containing calculations on distance and trajectory to the concertgoers below, according to a law enforcement source.

Lombardo said they have found "no evidence to show there was a second shooter."

Authorities have received 200 accounts of Paddock traveling around Las Vegas, and he "has never been seen with anyone else," the sheriff said.

Through her attorney, Paddock's girlfriend, Marilou Danley, said it "never occurred to me in any way whatsoever that he was planning violence."

What was he planning to do with the explosives in his car?

Investigators learned Paddock had "personal protection equipment with him," Lombardo told reporters Monday, leading authorities to believe he planned more than a mass shooting.

"We know he attempted to shoot at fuel tanks," Lombardo said. "I would be comfortable saying, depending on the splash he made during the shooting, it would enable first responders to draw their attention to other locations, which would allow Paddock to just leave the hotel."

Why did he do it?

Authorities have received more than 1,000 tips but still have no credible information on Paddock's motive.

Lombardo admitted the slow speed of the investigation frustrated him.

"It's because this individual purposely hid his actions leading up to this event and it is difficult for us to find the answers to those actions," he said.

Nothing about Paddock's life provides clues as to why he would have meticulously planned and perpetrated mass murder. The 64-year-old retired accountant had no apparent political or religious affiliations, one of his brothers said.

Paddock would wager up to $1 million a night but wandered around casinos in sweatpants and flip-flops. He carried his own drink into the high rollers' area because he didn't want to tip the waitresses much, according to a deposition obtained exclusively by CNN that was part of a 2013 civil lawsuit Paddock brought against the Cosmopolitan Hotel over a fall.

He had no criminal record. His purchase of an arsenal raised no red flags.

"Currently, we do not believe there is one particular event in the suspect's life for us to key on," the sheriff said.

In the 2013 deposition, Paddock said he had no mental issues or addictions.

Over the years, he owned apartments and houses in Nevada, Florida and California. Most recently, he lived in a retiree community in Mesquite, Nevada, northeast of Las Vegas.

He kept a low profile. Some neighbors say they rarely saw him.

In September, Paddock rented a room at a Las Vegas condo complex that overlooked the Life Is Beautiful music festival.