Investigators are sending Les Vegas gunman Stephen Paddock's brain off for further testing as they search for clues to explain why he killed 58 people two weeks ago.

The coroner completed Paddock's autopsy and has sent the body off to Stanford University where neuropathologists will put his brain under the microscope to check for signs of any hidden diseases, the New York Post reported.

Coroner John Fudenberg revealed the autopsy on the 64-year-old did not find anything out of the ordinary but confirmed that he wouldn't announce the result for several months.

Paddock's rain showed no signs of tumours, injuries or abnormality.

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"All those things that you would expect to find, we have not found," Clark County Sheriff Joe Lombardo said.

"We may never know" why Paddock chose to kill 58 people and injure more than 500 more, Lombardo said. But investigators are reportedly making progress each day.

Stephen Paddock's body, and specifically his brain, are being sent to Stanford for additional forensic analysis. Photo / Getty
Stephen Paddock's body, and specifically his brain, are being sent to Stanford for additional forensic analysis. Photo / Getty

It was revealed last week that Paddock took Valium at times for anxiety and had a doctor who prescribed it to him on retainer.

"He's like on retainer, I call it, I guess," Paddock said in a 2013 testimony for a civil lawsuit.

"It means I pay a fee yearly ... I have good access to him."

In the testimony he also revealed he was an avid gambler and described himself as the "biggest video poker player in the world" - betting as much as $US1 million a night.

While his testimony offers little insight into what could have prompted his killing spree, he said he had no mental health issues, criminal records or addiction habits.