Stephen Paddock: Las Vegas sheriff says gunman may have been 'radicalised unbeknownst to us'

'It was preplanned extensively and I'm pretty sure he evaluated everything that he did and his actions, which is troublesome'

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.

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.”

Additional reporting by agencies