Las Vegas gunman opened fire on a security guard six minutes before he rained down bullets on a crowd and killed 58 people, officials say, 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 on Monday. 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 Sheriff Lombardo, who added 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 October 1, killing 58 and wounding hundreds more, before shooting himself.

“In co-ordination 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,” Sheriff Lombardo said.

There is no indication anyone other than Paddock fired on the crowd, Sheriff 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, Sheriff 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 Mr Campos via surveillance cameras he set up outside his hotel suite, police have said.

Paddock shot the guard at 9.59pm local time, Sheriff 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 Mr Campos when they arrived on the floor.

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

Las Vegas police officer David Newton told CBS News program 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.

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

He added that Paddock shot at jet fuel tanks at McCarran International Airport. He had protective gear in the hotel suite and explosives in his parked car.

– AAP