Authorities have not yet found a motive for the October 2017 Las Vegas massacre but have concluded there is no evidence of any political or ideological radicalization that would explain why Stephen Paddock opened fire on a country music festival from a 32nd floor suite at the Mandalay Bay resort.

While the investigation remains active, the Clark County sheriff's office on Friday released a preliminary 81-page investigatory report about the Oct. 1 shooting rampage, which left 58 people dead and more than 850 injured. Paddock, who had checked into two rooms at the Mandalay and spent days bringing in bags of assault rifles and ammunition, shot down into the crowd for more than 10 minutes in what investigators have described as a well-planned attack.

Authorities have concluded that Paddock acted alone, and the sheriff said police do not anticipate bringing charges against Paddock's girlfriend, Marilou Danley, who received large cash transfers from Paddock just before the shooting and was questioned in the days after.

"We have done a lot of work trying to piece together what happened," Sheriff Joe Lombardo said during a news conference Friday. "This report won't answer every question, or even the biggest question, as to why he did what he did."

Paddock, 64, who had no prior criminal history, stockpiled weapons in the year before the shooting, ultimately purchasing 55 rifles and other guns in addition to scopes, cases, bump stocks and ammunition, according to the report. But it remains unclear why he targeted the concert, the central mystery that has gone unsolved since he opened fire on the Route 51 Harvest music festival in the deadliest mass shooting in modern American history.

"No suicide note or manifesto was found," investigators wrote. "There was no evidence of radicalization or ideology to support any theory that Paddock supported or followed any hate groups or any domestic or foreign terrorist organizations. Despite numerous interviews with Paddock's family, acquaintances and gambling contacts, investigators could not link Paddock to any specific ideology."

The report later concludes: "Nothing was found to indicate motive on the part of Paddock or that he acted with anyone else."

Unlike many of the other mass killers who have attacked churches, nightclubs, workplaces, schools and other public spaces across the country — and in many places left racist screeds or videotaped rants — Paddock apparently left no explanation for his attack.

Court filings released late last week showed that federal authorities believe he had planned meticulously for both the attack and for the investigation he knew would follow. The court documents, which detail the early days of the investigation, describe Paddock as spending significant time amassing weapons and ammunition while seeking "to thwart the eventual law enforcement investigation."

In one affidavit seeking a search warrant, an FBI special agent wrote that "the methodical nature of the planning employed by Paddock, coupled with his efforts to undermine the preceding investigation, are factors indicative of a level of sophistication which is commonly found in mass casualty events such as this."

Paddock moved guns and ammunition into his hotel suite in the days before the attack, sometimes getting help from the hotel bell desk while using a luggage cart and freight elevator. He set up cameras around the area so he could know when officers were closing in. According to police, Paddock shot a hotel security guard through his door not long before firing at the music festival across the street.

When police officers eventually breached Paddock's suite, they found him dead from a self-inflicted gunshot wound, a handgun not far from his body.

A search of Paddock's computers found an internet search history that included queries for information about outdoor concert venues, SWAT tactics, weapons and the locations of various gun stores. Authorities added that they have found "numerous" child pornographic photos on Paddock's computers and that federal officials are looking into a person of interest related to them.

Danley told investigators that Paddock had grown distant from her in the year before the shooting. Other friends and family members told police that Paddock claimed to consistently feel ill, in pain or fatigued, but Paddock's physician told investigators that Paddock's only major ailment was a muscle tear from a slip-and-fall incident in a casino three years before the shooting.

That doctor also told police that he believed Paddock "may have had bipolar disorder," but that Paddock did not want to discuss his mental health, "seemed fearful of medications" and refused anti-depression medication. The doctor said Paddock accepted prescriptions for an anti-anxiety drug, but it was unclear if he was taking it.

Investigators also concluded that Paddock was not facing financial hardship at the time of the shooting. He gambled extensively in the last days of September — wagering tens of thousands of dollars at the Mandalay and other Las Vegas casinos — but he also was traveling back to a home of his in Mesquite, Nev., where he made bank deposits, transferred money to his girlfriend and purchased more guns.

"He was indebted to no one and in fact paid all his gambling debts off before the shooting," investigators wrote.

The investigative report indicates Paddock was scoping out other concerts and venues from coast to coast before his attack in Las Vegas.

Two months before the shooting, in August, Paddock had booked a reservation at a Chicago hotel and specifically requested a room overlooking the outdoor venue of Lollapalooza, a major music festival. He canceled the reservation just two days before he was set to check in. His computers indicated searches for locations around Fenway Park in Boston and also about California beaches, including one that read: "how crowded does Santa Monica Beach get."

Paddock also booked hotel rooms in Las Vegas during the Life is Beautiful music festival, which was held in September, and was seen on surveillance camera footage transporting suitcases to and from the rooms.