Police on Oct. 9 revised their timeline of the Oct. 1 shooting in Las Vegas that left 58 people dead. (Video: Reuters)

Authorities investigating the Las Vegas massacre on Monday expressed irritation that after eight days, they have still not determined why a gunman opened fire on concertgoers, killing 58 people and injuring hundreds more in the deadliest mass shooting in modern American history.

“I’m frustrated,” Joseph Lombardo, the Las Vegas sheriff, said at a news briefing. “Because this individual purposefully hid his actions leading up to this event, and it is difficult for us to find answers for those actions.”

During the briefing, Lombardo offered a key update to the shooting timeline that officials previously laid out. Lombardo said that six minutes before the gunman opened fire — shooting at concertgoers far below his 32nd floor suite in the Mandalay Bay Resort and Casino — he first shot a hotel security guard and injured him.

Lombardo had previously said that while he did not know what caused Stephen Paddock, the 64-year-old gunman, to stop shooting, he assumed it was due to the security guard’s arrival.

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Police have warned that information released to the public could change as the investigation continues, and Lombardo said that this is what happened with the timeline involving the guard, Jesus Campos.

Speaking on Monday, Lombardo said Campos was shot at 9:59 p.m., six minutes before the gunman began raining bullets on the country music festival below. Campos arrived on the 32nd floor for an alarm involving an open door, Lombardo said. He said the door was not Paddock’s but did not elaborate.

Authorities do not know why Paddock stopped firing, Lombardo said Monday.

The updated timeline raises new questions about the law enforcement response to the shooting rampage. Even as Paddock began firing on the crowd — firing for 10 minutes — police did not know that the security guard was shot until they arrived on the hotel’s 32nd floor, Lombardo said. Campos alerted hotel security of the situation, Lombardo said, but police hunting for the gunman were not aware of the shots fired at the guard.

On Oct. 1, a crowd of thousands had gathered in Las Vegas for a music festival. Then bullets rained down from the sky. Survivors describe the shock and terror of the worst mass shooting in recent U.S. history. (Video: The Washington Post)

A representative of MGM Resorts, which operates the Mandalay Bay, did not respond to a request for comment Monday evening.

The new information comes as investigators are still struggling to explain why Paddock carried out the rampage. Authorities have pulled together a picture of Paddock’s mental state, Lombardo said, but have found no occurrence in his life that may have set him off.

“We do not believe there is one particular event in the suspect’s life for us to key on,” he said. “We believe he decided to take the lives he did and he had a very purposeful plan that he carried out.”

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Investigators are speaking to Paddock’s relatives and to his girlfriend, Marilou Danley, who returned to the country after the shooting and was interviewed by the FBI. They are also trying to scour his medical history, Lombardo said, and speak to people who knew him.

But so far, authorities looking to see whether anyone may have aided Paddock found no evidence of the gunman having associates in Las Vegas. Lombardo said investigators have discovered more than 200 instances of Paddock traveling through the city over an unspecified period of time, “and he has never been seen with anyone else.”

“We want to figure out the why to this, and we’d like to know the motive,” Lombardo said.

Lombardo said police still do not know what Paddock’s ultimate plans were, noting that the gunman shot at fuel tanks nearby and had explosives in his car.

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

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Officers who stormed Paddock’s room said they found an “armory” of guns and ammunition inside — along with a note suggesting the gunman had mapped out the trajectory of the bullets he would rain down on concertgoers.

Since the Oct. 1 massacre, while Paddock’s motive has baffled investigators, a picture of his life has taken shape. He was described as a man of considerable financial means who avidly played video poker and avoided human contact when possible. Paddock was meticulous and methodical, carefully plotting out his actions right up until the moment he shot himself in the head.

Before opening fire, Paddock had smuggled his arsenal into a two-room suite in Mandalay Bay and set up cameras to monitor police as they arrived.

The Las Vegas police officers who marched up to the hotel’s 32nd floor said in a television interview that they found further evidence of Paddock’s painstaking preparations as they approached. These officers, who at the time feared they might be dealing with multiple shooters, entered a hotel stairwell and discovered that Paddock had apparently blocked the door leading to the hallway near his suite.

“He had screwed shut the door — with a piece of metal and some screws,” Sgt. Joshua Bitsko, one of the officers, told “60 Minutes.” Dave Newton, another officer who responded, told “60 Minutes” that a third officer had “a pry bar and was able to easily pop that door.”

The Las Vegas police have given few media interviews since the massacre, instead releasing information through news briefings held over the past week. In the “60 Minutes” interview, broadcast Sunday night, officers offered new details about the shooting and what they encountered during the unfolding chaos.

Matthew Donaldson, a detective, said he was doing paperwork at police headquarters when the calls began coming in about the gunfire on the Las Vegas Strip. He sped to the scene but could only get so close and eventually had to take the last few blocks on foot while wearing cowboy boots.

“I took my boots off,” he said. “I just threw ’em in the casino. That was slowing me down. I was faster barefoot, and I was gonna be more effective barefoot.”

Survivors return to the scene in Las Vegas with the hopes of gathering belongings left behind in the chaos of one of the country's deadliest mass shootings. (Video: Alice Li, Dalton Bennett/The Washington Post, Photo: Salwan Georges/The Washington Post)

Police were told there was fire coming from someone on the 29th floor as well as the 32nd, the officers said, so they prepared to face multiple attackers. When they got through the barricaded stairwell door, the officers saw a hotel service cart with wires leading to Paddock’s suite and suspected it might have been rigged with explosives; they later discovered Paddock had placed a camera there.

Officers turned toward Paddock’s suite, seeing a door Newton said “was just riddled with bullet holes coming out. It looked like Swiss cheese.”

Unsure what they would find inside, the officers blew the door open, setting off the fire alarm. What they found inside “looked like almost a gun store,” Newton said.

“So many guns,” Newton said in the television interview. “So many magazines. Stacks and stacks of magazines everywhere. Just in suitcases all neatly stacked against pillars, around the room, all stacked up, rifles placed all throughout.”

Here's what you need to know about the gunman. (Video: Monica Akhtar/The Washington Post, Photo: Ethan Miller/The Washington Post)

The suite smelled of gunpowder and its floor was littered with shell casings, the officers said. They found monitors, laptops, phones, drills and drill bits — and, amid it all, Paddock’s body with a gun nearby. It was not clear when he shot himself.

Bitsko said the level of preparation and work inside, including setting up wires for the cameras, suggested that it took Paddock “days to finish” getting it all ready.

Officers then went through the room to make sure no one else was hiding behind a curtain or in the bathroom. Near where Paddock had opened fire, they also found a piece of paper, which was visible in images that have circulated online of the hotel suite. Scribbled on it were numbers, the officers said, that suggested Paddock had calculated his trajectory before opening fire.

“I could see on it he had written the distance, the elevation he was on, the drop of what his bullet was gonna be for the crowd,” Newton said. “So he had had that written down and figured out so he would know where to shoot to hit his targets from there.”

Matt Zapotosky contributed to this report, which was first published Monday evening and has been updated. 

Further reading:

Invisible wounds of Las Vegas shooting could affect tens of thousands

Victims of Las Vegas massacre struggle to recover from crippling injuries nearly a week later