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 three other cities including Chicago, Boston and Santa Monica, 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.

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.