After three months of investigation, police have learned that Stephen Paddock, who carried out the Las Vegas shooting in October, was a high-stakes gambler on a losing streak, obsessed with cleanliness, possibly bipolar and was having difficulties with his live-in girlfriend.

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But hundreds of interviews and thousands of pieces of evidence have not answered the key question: why did Paddock open fire from a high-rise hotel suite, killing 58 people and injuring more than 800 in the deadliest shooting in modern US history?

Sheriff Joe Lombardo of Clark County released a preliminary report on Friday on the 1 October attack on an outdoor concert staged from a room at the Mandalay Bay casino-hotel. He said he did not expect criminal charges to be filed against Paddock’s girlfriend, Marilou Danley, who had been named as the only person of interest in the case.

Investigators believe Paddock acted alone. He did not leave a suicide note or manifesto before he was found dead in the room.

Weeks before the shooting, the couple stayed at Mandalay Bay. Paddock was acting strangely, Danley told investigators. She remembered him constantly looking out of 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.

Paddock had become “distant” in the year before the shooting and their relationship was no longer intimate, Danley said in an interview with investigators. She described him as germophobic and said he had strong reactions to smells.

Paddock, who fatally shot himself in his hotel room, had told friends and relatives he always felt ill, in pain and fatigued.

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

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

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 said 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.

Investigators found 23 rifles and a handgun in Paddock’s hotel suite and more than a dozen of the rifles were 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.

In their report, police released never-before-seen photos from inside the hotel suite, including one shot through a broken window, showing the concert venue still littered with personal belongings.

Facebook Twitter Pinterest A photo released by Las Vegas police shows the interior of room 32-134 at the Mandalay Bay hotel. Photograph: Las Vegas Metropolitan Police De/AP

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

Before the attack, Paddock had researched Swat tactics and searched online for other potential public targets, including in Chicago, Boston and Santa Monica, California, the sheriff said.

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

A federal grand jury is hearing evidence in a case that spun off from the shooting investigation, the sheriff said. The FBI has “an ongoing case against an individual of federal interest”, Lombardo said, declining to elaborate.

Spokeswomen for the FBI and federal prosecutors in Las Vegas declined to comment.