Before Stephen Paddock opened fire from the 32nd floor of the Mandalay Bay hotel into a crowd of some 22,000 concertgoers in Las Vegas Sunday night, the man now responsible for the deadliest mass shooting in United States history grew up in Southern California and attended high school in Sun Valley.

Marilou Danley, the girlfriend of gunman Stephen Paddock, has become the focus of attention in the U.S. investigation into the Las Vegas mass shooting.

It's not clear at all what, if any, information she might be able to provide.

Danley lived with Paddock at a home in Mesquite, Nevada.

Almost 60 people have died in the attack, and more than 500 people were injured.

The gunman checked into the room days in advance, stocked a cache of weapons there and set up cameras inside his hotel suite and in the hallway.

Police don't know if the devices were transmitting - the FBI is investigating their use - but the Clark County sheriff said he thinks Paddock might have used them to watch for people approaching his room. She flew from Manila to Hong Kong on September 22 and returned to Manila on September 25 before flying to Los Angeles Tuesday.

Clark County Sheriff Joseph Lombardo told reporters Wednesday just after 9 a.m. local time that Danley would be questioned in the next half hour at the FBI's Los Angeles field office.

U.S. reports say that on top of his divorce proceedings, Stephen Paddock had a gambling problem.

Marilou Danley has been named as a person of interest but has not been charged with a crimeWho is Marilou Danley?

Former neighbors of Stephen Paddock didn't remember him ever talking about guns, but did notice his gambling.

As of now, Danley is not under arrest, nor is she charged with any crimes.

Danley, 62, was out the country at the time of the shooting and in the Philippines on Tuesday.

Authorities in the USA, the Philippines and Australia were involved in the search for Danley, who travels on an Australian passport, CNN reported.

They say that there have been more than 100 suspicious reports of Paddock transferring cash overseas.

Paddock ended Sunday night's shooting spree, the deadliest in modern US history, by killing himself.

Now on holiday in Japan with three girlfriends, Danley lived a luxury life in a scenic desert resort-like retirement village 100km north of where Sunday night's shooting took place.

The closest Paddock ever appeared to have come to a brush with the law was a traffic infraction, authorities said.

So far, police believe Paddock acted alone - which could make his motive harder to determine.

Employees at the Mesquite Starbucks said that after seeing his face so many times, it didn't take them long to recognize the man who's mugshot was soon plastered onto screens across the nation after Sunday's mass shooting at the Mandalay Bay Resort and Casino.

"Our family doesn't know Stephen Paddock, and has no knowledge of Marilou's relationship with Mr. Paddock".

At this point, Paddock's motive and aims remain a mystery to investigators. Earlier reports said that Danley was expected to return to US soil on Wednesday.