The Las Vegas gunman opened fire on a security guard six minutes before he rained down bullets on a crowd and killed 58 people, officials said on Monday in a change to the timeline of the deadliest mass shooting in modern US history.

The shooter, Stephen Paddock, 64, was seen on numerous occasions in Las Vegas without any person accompanying him and he gambled the night before the shooting, Clark County Sheriff Joseph Lombardo said at a news conference. He killed himself after the attack.

“This individual purposely hid his actions leading up to this event, and it is difficult for us to find the answers,” said Lombardo, who said he was frustrated with the speed of the investigation.

Paddock sprayed an outdoor concert with bursts of gunfire from high above in a Las Vegas hotel window on 1 October, killing 58 and wounding hundreds more, before shooting himself.

“In coordination with the FBI's behavioural analysis unit, a comprehensive picture is being drawn as to the suspect's mental state and currently we do not believe there is one particular event in the suspect's life for us to key on,” Lombardo said.

There is no indication anyone other than Paddock fired on the crowd, Lombardo said, adding investigators are talking to family members and the girlfriend of the gunman.

Paddock shot and wounded a security guard who came to his floor at the Mandalay Bay hotel to investigate an open door down near Paddock's suite, Lombardo said, providing new details on what occurred immediately before the mass shooting.

The security guard, Jesus Campos, was struck in the leg as the gunman, from behind his door, shot into the hallway on the 32nd floor. Paddock apparently detected Campos via surveillance cameras he set up outside his hotel suite, police have said.

Paddock shot the guard at 9.59pm local time, Lombardo said, shortly before raining down bullets on the Route 91 Harvest festival in an attack that began at 10.05pm and lasted 10 minutes.

Police officers found Campos when they arrived on the floor.

Paddock had a document in the room with him that contained numbers, Lombardo said, adding he could not immediately say what purpose the figures served.

Las Vegas police officer David Newton told CBS News programme 60 Minutes on Sunday that he entered the room and saw a note on the shooter's nightstand with numbers that appeared to be designed to help his aim.

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

It was unclear why Paddock stopped firing at the crowd, suggesting he may have initially planned to escape, Lombardo said.

He shot at jet fuel tanks at McCarran International Airport and had protective gear in the hotel suite and explosives in his parked car, Lombardo said.