LAS VEGAS (AP) —  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 made that announcement and revealed more about the investigation during a news conference on the shooting Friday morning.

Lombardo told reporters 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.

The sheriff did say that a federal grand jury is hearing evidence in a case spun off of the shooting, but he would not say whether anyone would be charged with a crime.

He also would not say what charges the grand jury is considering. All he would say is the FBI has "an ongoing case against an individual of federal interest."

Authorities say Paddock conducted extensive online searches for police and SWAT tactics and searched for other potential targets before he carried out the deadliest mass shooting in modern U.S. history. The sheriff said he had searched for several other public venues and took photographs of other potential sites.

A preliminary report about the shooting says Paddock searched for outdoor concert venues, the number of attendees at other concerts in Las Vegas and the number of people who go to the beach in Santa Monica, California.

The report also says Paddock also searched for information about several other hotels in Las Vegas.

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