Las Vegas police faced new questions on Tuesday about their response to this month's mass shooting after a county sheriff disclosed the gunman shot a security guard before, not after, opening fire on a crowd below his window at the Mandalay Bay hotel.

Las Vegas Metropolitan Police Department spokesman Aden Ocampo-Gomez said the office will respond later to questions involving police response time to the shooting that killed dozens of people at an outdoor concert.

On Monday, Clark County Sheriff Joseph Lombardo disclosed that gunman Stephen Paddock shot a hotel security guard responding to a door alarm near his 32nd-floor suite six minutes before he began firing out his window. Officials initially said Paddock shot the security guard after he began raining bullets down on the crowd.

“What we have learned is (the security guard) was encountered by the suspect prior to his shooting to the outside world,” Lombardo said.

Paddock, 64, killed 58 people and injured hundreds, before fatally shooting himself as police responded. Nine days later, his motive remained a mystery.

Lombardo did not address whether the mass shooting could have been prevented based on the new timeline, but said it was unclear why Paddock stopped firing on the concert.

In an active shooter situation, response time can be as fast as three minutes, said Sid Heal, a retired Los Angeles Sheriff's Department commander and tactical expert. He questioned why it took police so long to reach the room if hotel security immediately called them.

“Someone needs to account for those minutes,” he said.

Nevada Lieutenant Governor Mark Hutchison told CNN on Tuesday that there was a lot of information to evaluate, but acknowledged that Paddock did not stop firing because of the guard, Jesus Campos, as was initially assumed.

Campos immediately alerted the hotel's in-house security team after he was shot at 9.59pm, Lombardo said.

Police were not aware Campos had been shot until they met him in the hallway at 10.18pm, three minutes after Paddock had stopped firing on the concert. Police blew open Paddock's hotel room door 81 minutes after the shooting started.

Protocol for Las Vegas hotels and casinos is to barricade the corridor where a shooting takes place and wait for police to arrive, said David Shepherd, a security expert who advises the Las Vegas Metropolitan Police Department and who ran the security team at The Venetian hotel on the Vegas Strip for eight years.

Las Vegas shooting – in pictures Show all 15 1 /15 Las Vegas shooting – in pictures Las Vegas shooting – in pictures People scramble for shelter at the Route 91 Harvest country music festival after gun fire was heard Getty Las Vegas shooting – in pictures People carry a person at the Route 91 Harvest country music festival after shots were fired David Becker/Getty Las Vegas shooting – in pictures People run from the Route 91 Harvest country music festival after gun fire was heard David Becker/Getty Las Vegas shooting – in pictures A handout photo released via Twitter by Eiki Hrafnsson (@EirikurH) showing concertgoers running away from the scene (C) after shots range out at the Route 91 Harvest festival on Las Vegas Boulevard EPA/Eiki Hrafnsson Las Vegas shooting – in pictures People lie on the ground at the Route 91 Harvest country music festival after hearing gun fire Getty Las Vegas shooting – in pictures A man in a wheelchair is taken away from the Route 91 Harvest country music festival after hearing gun fire David Becker/Getty Las Vegas shooting – in pictures People stand on the street outside the Mandalay Bay hotel near the scene of the Route 91 Harvest festival on Las Vegas Boulevard EPA/Paul Buck Las Vegas shooting – in pictures FBI agents confer in front of the Tropicana hotel-casino after a mass shooting during a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Las Vegas police run by a banner on the fence at the Route 91 Harvest country music festival grounds after shots were fired David Becker/Getty Las Vegas shooting – in pictures An injured person is tended to in the intersection of Tropicana Ave. and Las Vegas Boulevard after a mass shooting at a country music festival Ethan Miller/Getty Las Vegas shooting – in pictures Metro Police officers pass by the front of the Tropicana hotel-casino after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures A woman sits on a curb at the scene of a shooting outside of a music festival along the Las Vegas Strip AP/John Locher Las Vegas shooting – in pictures A cowboy hat lays in the street after shots were fired near a country music festival in Las Vegas Getty Las Vegas shooting – in pictures Las Vegas Metro Police and medical workers stage in the intersection of Tropicana Avenue and Las Vegas Boulevard South after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Sheriff Joe Lombardo (2-R) speaking during a press briefing in the aftermath of the active shooter incident on Las Vegas Boulevard EPA

Police are trained to wait and negotiate with a shooter, rather than storm the room immediately, he said. The initial reports that night of multiple shooters at several hotels would also have confused police, he added.

“One of the biggest priorities is not to lose the life of a police officer,” Shepherd said by telephone. “So in those six minutes, it is highly unlikely police would have stormed that room.”

David Hickey, the president of the union that represents Campos, said that based on what he had heard, police and security officers acted as quickly as possible under the circumstances. Officials with MGM Resorts International, which owns the Mandalay Bay, declined to comment.