According to court records, Stephen Paddock was married and divorced twice. He was first married from 1977 to 1979, and for the second time from 1985 to 1990, both marriages in Los Angeles County, California. Family members say he stayed on good terms with his ex-wives. His brother Eric said that Stephen had no political or religious affiliations of any kind. Paddock lived in Texas and in California, then in a retirement community in Melbourne, Florida, from 2013 to 2015. In 2016, he moved from Florida to another retiree home in Mesquite, Nevada, about 80 miles (130 km) northeast of Las Vegas. According to property records, he bought a newly constructed single-family home in Mesquite in January 2015, and sold his two-bedroom home in Melbourne. Paddock lived in Mesquite for several years with his Filipino-born girlfriend whom he met at the Atlantis in Reno, Nevada. According to neighbors, they also lived together in Reno. Many Mesquite residents recalled only seeing him around town; those familiar with Paddock described him as someone who did not speak much and kept a low profile. The local gun community never saw him at any of the gun clubs or shooting ranges, including makeshift ones in the nearby desert. An Australian acquaintance said he met Paddock in the United States and in the Philippines. He described Stephen Paddock as intelligent and methodical. In his account, Paddock claimed to have won a lot of money using a betting strategy by applying algorithms to gambling on machines (not on tables). Paddock was conversant in gun laws and in defending his view of the Second Amendment. The acquaintance considered Paddock a generous man whenever he and his girlfriend visited him. In 2010, Paddock applied for and received a United States passport. He went on 20 cruise ship voyages, visiting several foreign ports including in Spain, Italy, Greece, Jordan, and the United Arab Emirates. He was accompanied by his girlfriend on nine of them. They went to the Philippines together in 2013 and 2014. During the last year of his life, they traveled on a cruise to the Middle East.

Stephen Paddock’s only recorded interaction with law enforcement was a minor traffic citation years before the shooting, which he settled in court. According to court records, Paddock also sued the Cosmopolitan of Las Vegas in September 2012, claiming he “slipped and fell on an obstruction on the floor” and was injured as a result; the lawsuit was dismissed with prejudice in October 2014. During his last months, he reportedly smelled of alcohol from early morning, and was despondent according to others. Stephen Paddock was reported to have filled prescriptions for the anti-anxiety drug Valium (brand name of diazepam), in 2013, 2016, as well as, the highest dose of 50 tablets 10-milligrams each in late June 2017. The chief medical officer of the Las Vegas Recovery Center said the effects of the drug can be magnified by alcohol, as confirmed by Dr. Michael First, a clinical psychiatry professor at Columbia University. Between October 2016 and September 28, 2017, Paddock purchased 33 firearms, the majority of them rifles, according to Bureau of Alcohol, Tobacco, Firearms and Explosives. At his suggestion, two weeks before the attack, his girlfriend went to her native Philippines. Paddock bought her a surprise airline ticket and soon later wired her $100,000 to buy a house there. He was spotted in Las Vegas with another woman, reported by investigators to be a prostitute. It has been confirmed that she is not an accomplice, and is not considered a suspect. Her name has not been released. Two days prior to the shooting, Paddock was recorded by a home surveillance system driving alone to an area for target practice located near his home.

On the night of October 1, 2017, at 10:05 p.m., Stephen Paddock opened fire from his hotel room on a large crowd of concertgoers at the Route 91 Harvest music festival on the Las Vegas Strip in Paradise, Nevada, killing 58 people and wounding 546 others. Paddock planned the attack meticulously. On September 25, six days beforehand, he checked in to a room on the 32nd floor of the Mandalay Bay Hotel, with 10 range bags and a computer. On September 28, he moved to the corner suite which overlooked the festival grounds, and stayed there for three days. There were 23 rifles and one handgun found inside his room by the police afterwards, including four DDM4 rifles, three FN-15 rifles, one AR-15 rifle with forward front grip, one .308-caliber AR-10 battle rifle, one AK-47, and at least one made-to-order LMT rifle, all very expensive, according to a law enforcement source. His arsenal included a large quantity of ammunition in special high capacity magazines, holding up to 75, or up to 100 cartridges each. Two of the rifles were resting on bipods, and were equipped with high-tech telescopic sights. Twelve of the weapons were outfitted with bump fire stocks, a recently-available firearms accessory that allows semiautomatic rifles to fire rapidly, simulating fully-automatic gunfire. Audio recordings of the attack indicated Paddock used these stocks to fire at the crowd in rapid succession.

One minute before the attack on the concertgoers, at 10:05 p.m., Stephen Paddock – who had placed a baby monitor camera on a service cart outside his room – fired about 200 rounds through his door, wounding an approaching hotel security guard The unarmed guard had attempted to enter the 32nd floor first at 9:59 p.m., on an unrelated matter, but found the door to the hallway barricaded. At 10:05 p.m. Paddock began firing hundreds of rounds in rapid succession at the crowd of thousands below. He stopped shooting ten minutes later at 10:15 p.m. It is unclear why. According to chronology of the events established by the authorities in the following days, the first two police officers reached the 32nd floor of the hotel at 10:17 p.m. and a minute later were shown the location of his door. Between 10:26 and 10:30 p.m. an additional eight LVMPD officers joined them and began clearing other suites along the 32nd floor hallway. At 10:55 p.m. eight SWAT team members entered the 32nd floor through the second stairwell nearest to Paddock’s suite. More than an hour after the first two officers arrived, at 11:20 p.m. the police breached his door with an explosive charge, and entered the room. Paddock was found dead inside his suite from a self-inflicted gunshot wound to the head In addition to the firearms and accessories found in Paddock’s hotel suite, there was a note that reportedly included handwritten calculations about where he needed to aim to maximize his accuracy. The note featured only the actual distance to the target, his own elevation, and the bullet trajectory relative to the line of fire.

During an investigation, ammonium nitrate, often used in improvised explosive devices, was found in the trunk of his car, along with 1,600 rounds of ammunition and 50 pounds (23 kilograms) of tannerite, a binary explosive used to make explosive targets for gun ranges. However, investigators clarified that while Paddock had “nefarious intent” with the material, he did not appear to have assembled an explosive device. An additional 19 firearms were found at his home. According to police, Paddock acted alone. His motive is unknown. The Islamic State of Iraq and the Levant claimed responsibility, but American law enforcement officials have given no evidence of a connection between Paddock and ISIS. No evidence has been found of any brain abnormality, according to autopsy results, though toxicology and other autopsy results have yet to be disclosed. Paddock’s brain was sent to Stanford University to receive a more extensive analysis.