Los Angeles (AFP) - The brother of the man behind the worst mass shooting in modern US history has been arrested on suspicion of having child pornography, police said on Wednesday.

Bruce Paddock -- one of Stephen Paddock's three brothers -- was arrested early Wednesday in Los Angeles, officer Norma Eisenman told AFP.

She said Paddock, 59, is facing child pornography charges in relation to a probe that began before the shooting in Las Vegas that left 58 people dead and hundreds wounded.

Bruce Paddock reportedly had a lengthy criminal history and over the years has been charged with various crimes including arson and burglary.

Police said he was arrested at an assisted living facility and was being held in lieu of $60,000 bail.

Authorities launched an investigation after finding evidence at a business where he had been squatting.

"At the time of the initial investigation, Paddock was a transient and could not be located," a statement said.

The Paddock brothers' father was on the FBI's most wanted list in the late 1960s after escaping from prison where he was serving time for bank robbery.