FBI Deputy Director Andrew McCabe said Wednesday that "we just don't know" the motives behind the gunman opening fire on thousands of concertgoers at a country music festival in Las Vegas on Sunday night.
Law enforcement officials were searching for clues Wednesday surrounding Stephen Paddock, the 64-year-old retired accountant and high-stakes gambler, who killed at least 58 people and left 527 others injured before killing himself.
"Anything is possible," said McCabe, who previously worked in the Counterterrorism Division of the FBI.
Officials don't have any immediately accessible thumbprints that would help identify the gunman's ideology or motivation for the shooting, McCabe said.
"Am I completely surprised? No, because this is an individual who is not on our radar or anyone's radar prior to the event. So we really have a challenging bit of detective work to do here to kind of put the pieces together after the fact," he said in an interview on "Squawk on the Street."
"That is, of course, what we're focused on," he said. "We're doing a lot of work in Las Vegas and many other places to get to the bottom of that."
Paddock's girlfriend, Marilou Danley, has arrived from the Philippines in Los Angeles, where FBI agents hoped to question her about the massacre, law enforcement officials told Reuters.
Paddock was not known to the federal authorities but was known to local law enforcement, according to NBC News. Police confirmed he was the son of bank robber Patrick Benjamin Paddock, who was on the FBI's Ten Most Wanted Fugitives list for over a decade until 1977.
President Donald Trump will reckon with the aftermath of a deadly mass shooting as he meets Wednesday with survivors and law enforcement officials.
—The Associated Press contributed to this report.