Las Vegas gunman Stephen Paddock may have been "radicalised unbeknownst to us", sheriff Joseph Lombardo has warned.

Authorities have been scrambling to find a motive for the 64-year-old's devastating attack on a country music festival on Sunday.

At least 58 people were killed and more than 500 injured when the retired accountant opened fire on revellers of the Route 91 Harvest festival from the 32nd floor of his Mandalay Bay hotel room before killing himself.

Mr Lombardo said the way Paddock had meticulously planned the massacre was "troublesome" after revealing he had set up cameras inside his hotel room and outside his door, apparently to spot anyone coming for him.

Police also recovered 23 weapons and "bump stock" devices — used to let rifles fire continuously — from Paddock's room on the Las Vegas Strip.

A further 19 guns were found in his house in Mesquite, along with what officers said were explosives, thousands of rounds of ammo and a supply of ammonium nitrate, which can be used to create explosives, in Paddock's car.

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

Mr Lombardo said he is “absolutely” confident authorities will find out what motivated Paddock, but the investigation is proceeding cautiously in case criminal charges are warranted against someone else.

“This investigation is not ended with the demise of Mr Paddock,” he said. “Did this person get radicalised unbeknownst to us? And we want to identify that source.”

He said of the cameras: “I anticipate he was looking for anybody coming to take him into custody."

“The fact that he had the type of weaponry and amount of weaponry in that room, it was preplanned extensively,” the sheriff added, “and I'm pretty sure he evaluated everything that he did and his actions, which is troublesome.”

One senior US homeland security official said there were no evidence that Paddock had links to international or domestic terror groups.

“We cannot even rule out mental illness or some form of brain damage, although there’s no evidence of that, either,” the official said.

His brother, Eric, has also been unable to provide any genuine insight into what led to the attack.

“It just makes less sense the more we use any kind of reason to figure it out,” he said in text message sent to Reuters. “I will bet any amount of money that they will not find any link to anything...he did this completely by himself.”