Las Vegas shooter believed to have had help, say investigators

United States investigators believe someone must have helped the Las Vegas shooter, as he planned his shooting spree that killed 58 concertgoers.

Stephen Paddock's girlfriend, Marilou Danley, says she knew something was going on, but just thought he was about to break up with her. 

The FBI has begun questioning Ms Danley, as they try to piece together why he planned the shooting and who knew about it.

She was met by federal agents at Los Angeles Airport, after returning voluntarily from the Philippines.

Paddock sent her there two weeks ago and wired her money to buy a house.

Investigators hope she'll help shed light on the motives behind the attack.

"To understand the 'why' helps you deal with the tragedy that we've all faced and we've all faced it, so believe me, we want to know why," says FBI special-agent-in-charge Aaron Rouse.

It's been revealed Paddock spent decades amassing weapons and ammunition. 

"He had to have some help at some point," Sheriff Joseph Lombardo says.

"We want to ensure that that's the answer...  maybe he's a super guy that was working out all this alone, but it would be hard for me to believe that."

Mr Lombardo says Paddock could have continued firing for much longer.  He had thousands more rounds in the hotel room, as well as boxes of explosives in his vehicle.

But speaking for the first time, through her lawyer, Ms Danley says she knew nothing of the attack.

"I was worried that, first, the unexpected trip home and then the money was a way of breaking up with me," attorney Matt Lombard said on her behalf. "It never occurred to me in any way whatsoever that he was planning violence against anyone."

Her distraught sisters told 7 News she was as confused as the rest of the world.

Police say they're determined to get to the bottom of the situation, no matter how long it takes.