An FBI official said on Thursday (NZ time) that no-one had been taken into custody over the shooting spree, but Sheriff Joseph Lombardo told the same news conference he found it hard to believe the arsenal of weapons, ammunition and explosives recovered by police could have been assembled by Paddock completely on his own.
The massacre was possibly in the planning stages for days, said investigators, as they struggled with a chilling but baffling array of clues yet were still left trying to grasp what caused a 64-year-old retiree to turn a concert ground into a killing field.
More than 22,000 people were attending the outdoor music festival when the gunman rained bullets from a high-floor hotel room of the Mandalay Bay Hotel and Casino on the Las Vegas Strip, police said.
At least 23 high-powered weapons were found in Stephen Paddock's hotel room.
Police had investigated Paddock's female companion Marilou Danley who was overseas when the shooting occurred.
He says Paddock fired on and off for nine to 11 minutes and unleashed a dozen or so volleys.
Ms Danley, 62, was not with him when he checked into the Mandalay, officers said.
"Marilou Danley is my sister, she is a good person and gentle soul".
"From the U.S. side, we are not able to comment on ongoing investigations, and I would not want to get ahead of the authorities leading the investigation", USA embassy spokeswoman Molly Koscina told AFP by email.
Marilou Danley said she feared Paddock was breaking up with her when he sent her overseas and gave her money to buy a home.
I have not made a statement until now because I have been cooperating with the authorities and I voluntarily flew back to America because I know that the Federal Bureau of Investigation and Las Vegas Police Department wanted to talk to me and I wanted to talk with them.
Sanders said that "we are monitoring the situation closely and offer our full support to state and local officials".
The brother also said that their father was bank robber Benjamin Hoskins Paddock who was on the FBI's 10 most wanted list until 1977.
In an interview with Australia's Channel 7 TV network, her sisters with their faces obscured and their names withheld, said they believe their sister couldn't have known about Paddock's plans.