This Oct. 2017 photo released by the Las Vegas Metropolitan Police Department Force Investigation Team Report showing the interior of room 32-134 of Las Vegas shooter Stephen Paddock's 32nd floor room of the Mandalay Bay hotel, an evidence imaged released as part of a preliminary report by Clark County Sheriff Joe Lombardo on Friday, Jan. 19, 2018, in Las Vegas. Paddock began shooting into the crowd attending the Route 91 Music Festival from his hotel room into a crowd of 22,000 people attending the Route 91 Harvest Festival music below on Oct. 1, 2017. (Las Vegas Metropolitan Police Department via AP)
This Oct. 2017 photo released by the Las Vegas Metropolitan Police Department Force Investigation Team Report showing the interior of room 32-134 of Las Vegas shooter Stephen Paddock's 32nd floor room of the Mandalay Bay hotel, an evidence imaged released as part of a preliminary report by Clark County Sheriff Joe Lombardo on Friday, Jan. 19, 2018, in Las Vegas. Paddock began shooting into the crowd attending the Route 91 Music Festival from his hotel room into a crowd of 22,000 people attending the Route 91 Harvest Festival music below on Oct. 1, 2017. (Las Vegas Metropolitan Police Department via AP) Credit: Las Vegas Metropolitan Police Department

Las Vegas — 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 on 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, 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.

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, Calif., 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.

On one of the computers found inside his hotel suite, 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 into a crowd of 22,000 people attending the Route 91 Harvest Festival music below, killing 58, Lombardo has said.

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.