FBI Deputy Director Andrew McCabe said Wednesday agency officials are looking to speak to “anyone and everyone” who may have crossed paths with the Las Vegas rampage gunman Stephen Paddock as investigators continue to search for answers about what led the retiree to carry out the deadliest mass shooting in U.S. history.

“This individual and this attack didn’t leave the sort of immediately accessible thumbprints that you find on some mass casualty attacks,” McCabe said while speaking at a cybersecurity forum in Boston.

McCabe added in an interview with CNBC that the gunman “is an individual who was 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 back together after the fact,” McCabe added.

The comments from the second-in-command at the FBI come as bureau investigators on Wednesday questioned the gunman’s girlfriend, Marilou Danley, with hopes of uncovering a motive for Sunday’s massacre that left 58 dead and more than 500 wounded.

In a statement she issued through her attorney, Matt Lombard, Danley suggested that there was nothing she witnessed in Paddock that could explain the ghastly violence.

“I knew Stephen Paddock as a kind, quiet, caring man,” Danley said. “I loved him and I hoped for a quiet future together with him. He never said anything to me or took any action that I was aware of, that I understood in anyway to be warning that something horrible like this was going to happen.”

Clark County, Nev. Sheriff Joseph Lombardo has described Danley — who was in the Philippines during the rampage — as “a person of interest." Danley arrived in Los Angeles on Wednesday morning and immediately met with federal agents.

A federal law enforcement official, who was not authorized to comment publicly, however, said Danley is not considered an accomplice, though investigators were expected to subject her to extensive questioning about the gunman’s activities prior to the assault, the extensive cache of weapons acquired by Paddock and the state of his finances.

She attended the interview with her attorney and was cooperative, the official said.

Can Las Vegas shooter's girlfriend answer the question, 'Why?'

Police say they found 23 guns in the 32nd floor Mandalay Bay Resort and Casino suite where Paddock carried out the assault, firing hundreds of rounds on concert-goers at a nearby country music festival. More weapons and ammunition were found at two homes owned by Paddock in Nevada. He had purchased 33 weapons in the last year alone, the ATF says.

Paddock also used surveillance cameras to monitor police approaches to his room at the Mandalay Bay — including a camera he positioned in the peephole of the door.

Authorities had been examining Paddock’s recent transfer of $100,000 to the Philippines prior to the deadly rampage.

Following her meeting with the FBI on Wednesday, Danley said in her statement that Paddock out-of-the-blue told her he had found a cheap ticket for her to travel to the Philippines and insisted she go visit her family there.

Once she was in the Philippines, Danley said Paddock wired her money to use to buy a home for her and her family.

“I was grateful, but honestly, I was worried that first the unexpected trip and then money was a way of breaking up with me,” Danley said in her statement. “It never occurred to me in anyway whatsoever that he was planning violence against anyone.”