Clark County Sheriff Joe Lombardo speaks at a news conference, Aug. 3, 2018, in Las Vegas.
Clark County Sheriff Joe Lombardo speaks at a news conference, Aug. 3, 2018, in Las Vegas.

LAS VEGAS - Officials closed their investigation Friday of the deadliest mass shooting in modern U.S. history, concluding the gunman acted alone, with no discernible motive.

Stephen Paddock, who in October 2017 killed 58 people and injured over 800 more by firing into a concert crowd gathered below his hotel room on the Las Vegas Strip, was an otherwise "unremarkable man" with a troubled mind, Clark County Sheriff Joe Lombardo said at a press conference Friday.

FILE - Photos and notes adorn a wall at the Las Ve
FILE - Photos and notes adorn a wall at the Las Vegas Community Healing Garden in Las Vegas, Oct. 16, 2017. The garden was built as a memorial for the victims of the mass shooting in Las Vegas.

Paddock was found dead in his hotel room, at the Mandalay Bay resort, of an apparent self-inflicted gunshot wound following the shooting. He was 64.

"What we have been able to answer are the questions of who, what, when, where and how," Lombardo said. "What we have not been able to definitively answer is ... why Stephen Paddock committed this act."

FILE - This undated photo provided by Eric Paddock
FILE - This undated photo provided by Eric Paddock shows his brother, Las Vegas gunman Stephen Paddock. Stephen Paddock opened fire on the Route 91 Harvest Festival, Oct. 1, 2017, killing dozens and wounding hundreds.

The 10-month investigation found that Paddock's friends and family reported him growing increasingly unstable in the months leading up to the shooting.

Bruce Paddock, Stephen's brother, said he believed Stephen had "mental illness and was paranoid and delusional." And Stephen Paddock's doctor thought he could be bipolar, authorities said.

Marilou Danley, Paddock's girlfriend, told investigators that Paddock's doctors had told him he had a "chemical imbalance." Danley also said Paddock would refuse to shake hands with others.

Danley was in the Philippines at the time of the shooting, and said she had no knowledge of her boyfriend's plans. Authorities had earlier announced they did not plan to bring charges against Danley.

"Paddock was described by many as a narcissist and only cared about himself," the report read. "[He] needed to feel important and only cared how relationships would benefit him."

Paddock's financial records also revealed he had lost $1.5 million in the two years leading up to the attack. Earlier reports had characterized Paddock as a frequent gambler, and patron of the casinos of Las Vegas.

The Las Vegas Police Department also announced that no further charges would be sought in connection with the attack.

In February, an Arizona man named Douglas Haig was charged with selling Paddock armor-piercing bullets without a license to do so. Haig, who has pled not guilty, is the only person to have charges brought against them in connection with the attack.

The Federal Bureau of Investigation is expected to release a psychological profile of Paddock later this year, Lombardo said.