LAS VEGAS — FBI agents questioned the Las Vegas gunman’s girlfriend on Wednesday as they struggled to get inside the mind of Stephen Paddock, a frustratingly opaque figure who carried out his high-rise massacre without leaving the plain-sight clues often found after major acts of bloodshed.

Three days after Paddock gunned down 59 people, Marilou Danley was interviewed at the FBI’s office in Los Angeles and had an attorney with her, according to a law enforcement official who was not authorized to discuss the case publicly and spoke on condition of anonymity.

Danley, 62, who has been called a “person of interest” by investigators, was met by federal agents Tuesday night when she arrived at the Los Angeles airport from her native Philippines after more than two weeks abroad.

Investigators are busy reconstructing Paddock’s life, behavior and the people he encountered in the weeks leading up to the deadliest mass shooting in modern U.S. history, FBI Deputy Director Andrew McCabe said. That includes examining his computer and cellphone.

But as of Wednesday, investigators were unable to explain what led Paddock to rain heavy fire down on a country music festival Sunday night from the windows of his 32nd-floor room at the Mandalay Bay hotel casino. He killed himself as police closed in. The attack left more than 500 people injured.

“This individual and this attack didn’t leave the sort of immediately accessible thumbprints that you find on some mass casualty attacks,” McCabe said.

The 64-year-old retired accountant quietly stockpiled an arsenal of high-powered weapons while pursuing a passion for high-stakes gambling at Nevada casinos, where his game of choice was video poker, a relatively solitary pursuit with no dealer and no humans to play against.

Neighbors described Paddock as friendly, but he wasn’t close to them.

“He was a private guy. That’s why you can’t find out anything about him,” his brother, Eric Paddock, said from his home in Florida. As for what triggered the massacre, the brother said: “Something happened that drove him into the pit of hell.”