Read the latest on the Las Vegas shooting with Friday’s live updates.

LAS VEGAS — The man who killed 49 people at an Orlando nightclub last year pledged allegiance to the Islamic State, in a 911 call, as the massacre unfolded. The sniper who shot to death five police officers in Dallas told the police that his goal was to attack white people. The man who attacked a black church in Charleston posted a racist manifesto online.

In one mass shooting after another, gunmen have offered telling evidence of their motives: complaining of “baby parts” after a shooting at Planned Parenthood, sympathizing with the Islamic State with a Facebook post on the day of the San Bernardino shooting, asking members of Congress if they were Republicans before pulling the trigger at a congressional baseball practice.

But in the four days since Stephen Paddock’s attack in Las Vegas — a shooting rampage that left 58 dead and hundreds seriously wounded — what drove him has remained a mystery, vexing the public and putting enormous pressure on federal and local investigators to find answers.

“In the spirit of the safety of this community or anywhere else in the United States I think it’s important to provide that information, but I don’t have it,” Sheriff Joseph Lombardo of the Las Vegas Metropolitan Police Department said in an interview Thursday. “We don’t know it yet.”

No grandiose manifesto has been found. No account of Mr. Paddock behaving dangerously or holding extremist views has emerged from neighbors or relatives. Unlike past killers, Mr. Paddock did not dial up the police to explain his actions.

The F.B.I. took Mr. Paddock’s computers and cellphones to its laboratory in Quantico, Va., for review, law enforcement officials said. Agents interviewed his girlfriend, Marilou Danley, in an attempt to determine his mental state at the time of the shooting, but Sheriff Lombardo said he was “not at liberty to say” what information had been learned.

Of course, investigators could at any time come across evidence that reveals Mr. Paddock’s thinking. “I’m pretty confident we’ll get there,” Sheriff Lombardo said.

Agents have fanned out across the country, interviewing family members and friends and looking for signs of mental illness.

Mr. Paddock left a trail of clues that are, so far, more cryptic than revealing: There was a note in his hotel room whose exact contents the authorities have yet to reveal. Sheriff Lombardo said that it contained numbers that were being analyzed for their relevance, and that it was not a manifesto or suicide note.

Mr. Paddock may have scouted other locations, including Fenway Park in Boston, Lollapalooza in Chicago and the Life is Beautiful music festival in Las Vegas, before finally checking into a suite at the Mandalay Bay that had clear sight lines to Route 91, and a massive gathering of country music fans. He stockpiled expensive firearms over the course of many months.