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.