Authorities say Danley, who lived with Paddock at his home in a Mesquite, Nevada, retirement community where officials searching the residence found a number of weapons, is more than a mere witness.
A police official revealed she voluntarily returned to the United States in a bid to "clear her name" of any involvement in the massacre.
Although police said they had no other suspects, Clark County Sheriff Joseph Lombardo said investigators wanted to talk with Paddock's girlfriend and live-in companion, Marilou Danley, who he said was traveling overseas in Tokyo. She was accompanied by the Federal Bureau of Investigation in Los Angeles, where Las Vegas police planned to question her, the source said.
Las Vegas Sheriff Joe Lombardo said Tuesday his investigators were hoping to speak with the 62-year-old woman.
Danley's sisters, speaking to CNN affiliate Seven Network Australia, said that Danley didn't know anything about the shooting ahead of time and that Paddock "sent her away" before the attack.
She was located in Japan on Tuesday (NZ time) a day after Paddock filled his 32nd floor suite of the Mandalay Bay casino with 19 high-powered guns, including assault rifles with scopes, and fired on the crowd below.
She married American Geary Danley in 1990, and they registered their marriage in Nevada.
Danley is a citizen of Australia originally from the Philippines and still has family there, Australian Foreign Minister Julie Bishop said.
Paddock and Danley shared a home in the town of Mesquite, 80 miles (130 kilometers) northeast of Las Vegas, where the authorities found a number of guns while searching the property.
No one knows why Paddock morphed from a retired accountant to the deadliest mass shooter in modern USA history.
The casino said in a statement she stopped working there "several years ago" and that it had no information on the pair.
Paddock seemed unlike the troubled, angry young men who experts said have come to embody the mass-shooter profile in the United States.
After a mass shooting, or any shooting really, if the murderer happens to be a white dude, he usually gets the "oh he was always so nice and quiet, I can't believe he would do something like this" treatment.
The evidence laid out by investigators so far shows Paddock meticulously planned the shooting.
At this point, Paddock's motive and aims remain a mystery to investigators.
The sheriff declined to confirm reports Paddock wired $US100,000 to the Philippines before the shooting.
Public records on Paddock point to an itinerant existence across the West and Southeast parts of America, including stints as an apartment manager and anaerospace industry worker. They said they want people to know that social media has spun this out of control and jumped to conclusions that their father was involved.