The Las Vegas gunman meticulously planned how to carry out the worst mass shooting in modern U.S. history, researching SWAT tactics, renting other hotel rooms overlooking outdoor concerts and investigating potential targets in at least four cities, authorities said Friday.

But almost four months after Stephen Paddock killed 58 people and wounded more than 800 others with a barrage of bullets from the Mandalay Bay casino-hotel, investigators still have not answered the key question: Why did he do it?

Clark County Sheriff Joe Lombardo released a preliminary report on the Oct. 1 attack and said he did not expect criminal charges to be filed against Paddock's girlfriend, Marilou Danley, who had been called a person of interest in the case.

Paddock, who killed himself before police reached him, told friends and relatives that he always felt ill, in pain and fatigued, authorities said.

His doctor thought he may have had bipolar disorder but told police that Paddock refused to discuss the possibility, the report said. The doctor offered him antidepressants, but Paddock accepted only a prescription for anxiety medication. He was fearful of medication and often refused to take it, the doctor told investigators.

During an interview with authorities, Paddock's girlfriend said he had become "distant" in the year before the shooting and their relationship was no longer intimate.

When they stayed at the Mandalay Bay together in September 2017, Paddock acted strangely, she told investigators. She remembered him constantly looking out the windows overlooking an area where the concert would be held the next month. He moved from window to window to see the site from different angles, the report said.

She described him as "germaphobic" and said he had strong reactions to smells.

The 64-year-old retired accountant was a high-stakes gambler and real estate investor. He had lost a "significant amount of wealth" since September 2015, which led to "bouts of depression," the sheriff has said. But Paddock had paid off his gambling debts before the shooting, according to the report.

Prior to the attack, Paddock's online searches included research into SWAT tactics and consideration of other potential public targets, including in Chicago, Boston and Santa Monica, California, the sheriff said.

His research also sought the number of attendees at other concerts in Las Vegas and the size of the crowds at Santa Monica's beach.

Among his searches was: "do police use explosives," the report said.

Four laptops and three cellphones were found inside his hotel suite. On one of the computers, investigators found hundreds of photos of child pornography.

The same computer was used to search online for the height of the Mandalay Bay, how to remove hard drives from laptops, the locations of gun shows in Nevada and information about several other Las Vegas casinos.

Authorities have said they found no link to international terrorism.

Paddock fired more than 1,100 bullets, mostly from two windows on the 32nd floor of the Mandalay Bay hotel, into a crowd of 22,000 people attending the Route 91 Harvest Festival music below, Lombardo has said.

That includes about 200 shots fired through his hotel room door into a hallway where an unarmed hotel security guard was wounded in the leg and a maintenance engineer took cover.

Several bullets hit fuel storage tanks at nearby McCarran International Airport that did not explode. Authorities reported finding about 4,000 unused bullets in Paddock's two-room suite, including incendiary rounds that Lombardo said were not used.

Investigators found 23 guns in the rooms, including 12 rifles fitted with "bump stock" devices that allowed rapid-fire shooting similar to fully automatic weapons. Dozens of guns were strewn around the room, some left inside a bassinet.

Danley was in the Philippines at the time of the shooting. In the days before the attack, Paddock sent her a $100,000 wire transfer. She has said she found that odd and thought he might have been breaking up with her when he sent her the money and told her to use it to buy a home for her family there.

During an interview with the FBI after she returned from the Philippines, Danley volunteered that investigators would find her fingerprints on bullets used during the attack because she would sometimes help Paddock load high-volume ammunition magazines, according to FBI warrant documents.

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Balsamo reported from Los Angeles.