Investigators say they have still not determined why Las Vegas mass shooter Stephen Paddock opened fire from his high-rise hotel suite, killing 58 people and injuring hundreds.

Clark County Sheriff Joe Lombardo told reporters Friday that investigators believe Paddock acted alone in the Oct. 1 shooting at the Mandalay Bay casino-hotel.

Lombardo says he does not expect charges to be filed against Paddock's girlfriend, Marilou Danley, who had been previously called a person of interest in the case.

Lombardo said the investigation found that Paddock had possessed child pornography.

Clark County Sheriff Joe Lombardo's briefing comes three days after a lawyer for the police department asked a Nevada state court judge to keep search warrant documents sealed because charges are still being investigated. The judge agreed to wait until at least next week.

A department spokeswoman said criminal charges could relate to items discovered in the service of warrants at homes, vehicles, cellphones, internet and email accounts belonging to the shooter, Richard Paddock, and his girlfriend, Marilou Danley.

Lombardo is the elected head of the metropolitan police department. His last media briefing about the worst mass shooting in modern U.S. history was Oct. 13, following several days of shifting timelines and accounts. The sheriff has expressed frustration about speculation and theories that have grown about the shooting and an investigation that he compared with solving a puzzle.