The Las Vegas sheriff leading the investigation into America's worst mass shooting has said 'we may never know' what motivated Stephen Paddock to kill 58 people.

Joe Lombardo said an exam of Paddock's brain has revealed 'no abnormalities' while girlfriend Marilou Danley had 'no concerns' over his mental health.

Detectives have contacted every branch of Paddock's family tree, including both his ex-wives, but have no uncovered anything that would explain his murderous actions.

Sheriff Jo Lombardo said an exam of Stephen Paddock's brain revealed 'no abnormalities' that would have led to the massacre, while his girlfriend had 'no concerns' for his mental health

Lombardo said 'we may never know' what led Paddock to kill 58 people, adding that extensive investigations of his family and friends have not thrown up any red flags

'All those things that you would expect to find, we have not found,' Lombardo said in an interview with the Las Vegas Review-Journal.

Asked if investigators are any closer to uncovering a motive, he added: 'We may never know.'

Lombardo was able to shed more light on Paddock's time in Las Vegas before he began his massacre on Sunday, October 1.

Paddock had arrived in the city three days earlier than originally thought, but stayed in a different room that was provided free of charge by the hotel.

When he moved to the corner suite from which he carried out the massacre, the hotel began charging him for the stay.

Police initially only counted charged days, hence the confusion, Lombardo said.

Lombardo said his team had reviewed hours of CCTV footage, and were not aware of any evidence to suggest he went into the desert to practice shooting in the days before the massacre, or that he hired a prostitute, as has been reported.

While Lombardo was cautious about giving an exact timeline, he reiterated that Paddock shot a Mandalay Bay security guard before opening fire on a country music festival

Paddock used an arsenal of 23 weapons to kill 58 people at the Route 91 Harvest Festival before killing himself as officer breached his hotel room

A bellboy did help Paddock move several bags up to his room, Lombardo revealed, adding that the killer traveled back and forth to his hometown of Mesquite several times before the massacre.

Lombardo confirmed that a Dr Winkler had treated Paddock for anxiety, and said he was not aware of any other medics treating him.

It is thought that Dr Winkler proscribed Paddock Valium, which can trigger underlying aggressive behavior to surface.

Lombardo said toxicology tests have not come back and the autopsy on Paddock is still underway.

The sheriff also revealed that during his rampage, Paddock shot a Mandalay Bay security guard in the thigh before pumping 200 bullets into the surrounding rooms, though fortunately nobody else was hurt.

It was the guard that alerted police to the location of the shooter, he said.

Paddock used an arsenal of 23 weapons to shoot 58 people dead, before killing himself as police breached the door to his room.