Drapes billow out of broken windows on Oct. 2 at the Mandalay Bay Resort and Casino on the Las Vegas Strip following the massacre. (John Locher/AP)

When the Las Vegas gunman arrived at the Mandalay Bay Resort and Casino late last month, he assembled "an armory" in his room, according to one of the police officers who went inside.

Authorities said Stephen Paddock, 64, had secreted nearly two dozen guns in his two-room suite on the 32nd floor. He also had stacks of ammunition, cameras, drills and computers as he methodically planned and carried out the deadliest mass shooting in modern American history.

But the first shots Paddock fired were not aimed at the country music festival far below. Instead, police now say, six minutes before Paddock aimed his guns at the concertgoers, he shot a hotel security guard who happened into his hallway on an unrelated call.

The new detail, which contradicts a significant element of the earlier timeline offered by police in the days since the shooting, raises additional questions about the law enforcement response to the massacre and why officers took as long as they did to arrive at Paddock's gunshot-riddled door.

These questions come as investigators, who have scoured Paddock's life in search of a motive since the Oct. 1 rampage, remain unable to explain why the avid gambler killed 58 people and injured hundreds more before shooting himself in the head.

"I'm frustrated," Joseph Lombardo, the Las Vegas sheriff, said at a news briefing Monday night. "This individual purposefully hid his actions leading up to this event and it is difficult for us to find answers for those actions."

Lombardo said that police are still unable to explain why the gunman stopped firing from his suite after 10 minutes of sustained volleys into the concert crowd.

Police previously said that the guard, Jesus Campos, was wounded during the shooting, and Lombardo had said that while he did not know for sure, he thought that Paddock saw Campos approaching and "was in fear that he was about to be breached."

Information released immediately after shootings or other mass-casualty attacks is often not firm while officials sift through reports and rumors in search of facts; it often takes time, for example, to dispel reports of multiple shooters or secondary attacks.

In Las Vegas, though, details released by law enforcement have remained remarkably fluid, illustrated by the revelation Monday — eight days after the shooting — that the guard was injured before, not during, the concert shooting.

The new detail raises still more questions, including when reports of the initial shots were relayed to others before and during the massacre to follow. Lombardo said Campos "immediately" notified hotel security after being shot, and he said police — who were searching the hotel for the source of the gunfire — did not know Campos had been hit until emerging on the 32nd floor.

An investigator works in the room at the Mandalay Bay Resort and Casino where the gunman opened fire, killing 58 people below at an Oct. 1 concert. (Gregory Bull/AP)

It was not clear whether hotel security officials told police that Campos had been shot and whether that information was then relayed to officers searching the hotel — potentially critical information as the shooting unfolded. An MGM spokeswoman expressed doubts about the timeline offered by Lombardo.

"As evidenced by law enforcement briefings over the past week, many facts are still unverified and continue to change as events are under review," Debra DeShong, the spokeswoman, said in a statement Tuesday night. "We cannot be certain about the most recent timeline that has been communicated" publicly, she said, "and we believe what is currently being expressed may not be accurate."

DeShong said "it is not appropriate for us to comment further at this time on what remains an open matter for law enforcement."

Las Vegas police did not respond to a list of questions regarding the timeline and declined to make Lombardo available for an interview.

In his briefing, Lombardo called the updated timeline a "minute change" that he said emerged during the investigation.

"As I have conveyed to you from the very beginning, in your zest for information, in my zest to ensure the public safety, the calming of their minds, is some things are going to change," Lombardo told reporters.

Exactly when information about the security guard being shot was relayed — and to whom — could be crucial as other police departments, in an effort to train their officers for a mass shooting, look to what happened in Las Vegas for lessons.

Similar exercises have followed high-profile mass shootings across the country, including those in San Bernardino, Calif., and at the Washington Navy Yard.

Experts say that the main focus for law enforcement during a shooting is to find and eliminate the attacker.

"Once they determine an active shooting is going on, the first priority is to stop the killing," said Peter Blair, a criminal justice professor and executive director of the Advanced Law Enforcement Rapid Response Training Center at Texas State University. "The longer delay there is in finding that person, the more time that person has to operate, to shoot people, to cause damage and chaos. So it's important to keep that window as small as possible."

Blair, who co-authored a 2013 FBI study examining 160 mass shootings, cautioned that it is unclear precisely when and how information was relayed in Las Vegas. But he said that chaos is common during mass attacks.

"Having seen a lot of these events, there's always a lot of confusion going on," Blair said. "The police, once the attack starts, dispatch is drowning in this information, you hear all these reports of gunshots going on and the confusion about what is going on is very, very common."

Lombardo said that Campos was shot at 9:59 p.m. after heading to the 32nd floor in response to an alarm reporting an open door to another room. Paddock saw the guard on cameras he had stashed in his door and the hallway, Lombardo said last week, and shot Campos, who survived.

Paddock then began firing into the crowd at 10:05 p.m. and stopped 10 minutes later, according to a police timeline. Authorities have said that Paddock used devices called bump stocks to turn at least a dozen rifles into something akin to automatic weapons.

Bump stocks have become the focus of controversy since the shooting, and a prominent gun-control group on Tuesday filed a lawsuit against a manufacturer of these devices.

During the shooting, two officers searching on the 31st floor heard gunfire above them, Lombardo said. When they arrived on the 32nd floor at 10:17 p.m., Campos told them that he was shot and directed them toward Paddock's room.

"They weren't aware of him being shot until they met him in the hallway after exiting the elevator," Lombardo said Monday.

Police have called Campos a hero and Lombardo on Monday described the guard as pivotal in helping "pinpoint the location of the suspect." He also said that after Campos was shot, a maintenance worker arrived on the floor, but the guard "prevented him from receiving any injuries."

Lombardo said Monday he would not assume that seeing Campos's approach prompted Paddock to speed up his shooting timeline, though he acknowledged that police remain unaware of what the gunman's ultimate plans were.

The sheriff noted that Paddock had explosives in his car and said he apparently tried to shoot at large fuel tanks near the concert venue.

"We do not know whether he had planned to cause additional harm outside of what happened at Mandalay Bay," Lombardo said.

Officers who arrived at Paddock's suite cleared the other rooms on the floor and did not rush into the suite because he had stopped firing, so they viewed the situation as a barricade rather than an active shooting, Lombardo said during an earlier briefing.

When the officers breached the room at 11:20 p.m., they found Paddock dead on the floor with a gun nearby.