Police sought clues on Tuesday to explain why a retiree with a penchant for gambling but no criminal record set up a sniper’s nest in a high-rise Las Vegas hotel and poured gunfire onto a concert below, slaying dozens of people before killing himself.

The Sunday night shooting spree from a 32nd-floor window of the Mandalay Bay hotel, on the Las Vegas Strip, killed at least 59 people before the gunman turned a weapon on himself. More than 500 were injured, some trampled, in the deadliest mass shooting in modern U.S. history.

The gunman, identified as Stephen Paddock, 64, left no immediate hint of his motive for the arsenal of high-powered weaponry he amassed, including 34 guns, or the carnage he inflicted on a crowd of 22,000 attending an outdoor country music festival.

Paddock was not known to have served in the military, or to have suffered from a history of mental illness or to have registered any inkling of social disaffection, political discontent or radical views on social media.

U.S. officials also discounted a claim of responsibility by the militant Islamic State group.

“We have determined to this point no connection with an international terrorist group,” Aaron Rouse, special agent in charge of the Federal Bureau of Investigation field office in Las Vegas, told reporters on Monday.

Police said they believed Paddock acted alone but were at a loss to explain what might have precipitated it.

“We have no idea what his belief system was,” Clark County Sheriff Joseph Lombardo told reporters. “I can’t get into the mind of a psychopath.”

Although police said they had no other suspects, Lombardo said investigators wanted to talk with Paddock’s girlfriend and live-in companion, Marilou Danley, who he said was traveling abroad, possibly in Tokyo.

Lombardo also said detectives were “aware of other individuals” who were involved in the sale of weapons Paddock acquired.