The man behind the mass shooting in Las Vegas shot and injured a security guard six minutes before carrying out the attack, officials say, in a major change to the timeline of events.
Police initially said the guard interrupted Stephen Paddock as the attack was under way.
The revelation is likely to raise questions why police were unable to locate Paddock earlier.
The attack left 58 people dead and was the worst in modern US history.
Paddock sprayed gunfire from the upper floors of the Mandalay Bay hotel on to a concert attended by thousands of people.
The guard – Jesus Campos – had been credited with distracting the gunman from killing more people because he was forced to shoot him through the door of the hotel room.
Police said at the time that after injuring the guard, Paddock did not fire on anymore into the crowd.
But now police say that Paddock shot Campos in the leg on 9:59 pm on 1 October. They say the guard was investigating an alarm that went off in another room on the same floor.
‘Some things were bound to change’
Paddock began firing on the crowd at 10:05 pm, police say, and continued for up to 11 minutes more.
Clark County Sheriff Joseph Lombardo said that police did not realise that the security guard had been shot until they arrived on the 32nd floor of the Mandalay Bay hotel. That was the case even though the guard managed to alert the hotel’s security team.
Mr Lombardo said new information had emerged during the investigation and that “some things were bound to change” given its complicated nature.
Police also say that the gunman, 64, was seen on numerous occasions in Las Vegas gambling heavily by himself the night before the shooting, which ended with him killing himself.
Paddock was dead by the time officers entered the room and it is not known why he had stopped shooting.
“This individual purposely hid his actions leading up to this event, and it is difficult for us to find the answers,” Mr Lombardo told a news conference. Paddock’s motives remain unclear.
Extensive research into the gunman’s mental state found no occurrence in his life that may have triggered such a violent turn of events, police say.
It is suspected Paddock scouted out other locations near open-air concerts in the weeks before the Las Vegas attack.