On a warm Sunday night exactly six months ago, Stephen Paddock barricaded himself in the Mandalay Bay hotel, assembled a makeshift video surveillance system, smashed two windows of his suite on the 32nd floor, loaded his rifles and opened fire on 22,000 concertgoers who had assembled on the Las Vegas Strip below. It is the worst mass shooting in modern U.S. history, but what inspired it?

Paddock, a 64-year-old gambler, retired accountant and real estate investor, fired more than 1,100 rounds rounds into the crowd at the Route 91 Harvest music festival, killing 58 people and injuring more than 800 others in just 10 minutes. About an hour after he fired his last shot, police found Paddock in his suite, dead of a self-inflicted gunshot wound. He didnt leave a suicide note.

The investigation into the shooting began just hours after the massacre. In the months that followed, the Las Vegas Metropolitan Police Department and the FBI reviewed more than 21,000 hours of video and 200,000 images, and conducted interviews with at least 43 people. Every facet of Paddocks life was explored, investigators said in a January preliminary report , the only one to have been released on their findings so far.

Agents determined that Paddock had meticulously planned the shooting spree. He amassed an arsenal of firearms and thousands of rounds of ammunition. He searched the web for summer concerts 2017 and biggest open air concert venues in USA. They searched for clues about what may have motivated him to kill that night: Was it a longstanding gambling habit? A relationship with his girlfriend that was growing increasingly estranged? An act of terrorism inspired by the Islamic State? A sudden change in his behavior?

A Self-Funded Gambling Habit And A Puzzling Claim

FBI investigators tend to look at a crime from four key perspectives to try to determine a criminals motives, explained Andrew Bringuel, a former member of the FBIs behavioral analysis unit who is not involved in the Paddock investigation. When assailants have political or social motives, they often want their viewpoints known to the world. They may leave a manifesto or a statement outlining their reasoning or beliefs. When personal or economic motives are at play, the trail of evidence may be less overt, but investigators can find clues in financial statements or family histories. Criminals can also be driven by several motives at the same time.

Paddock did not leave a political statement or manifesto not at the scene, not in his home, not amid his personal belongings. But mere hours after the attack, the self-described Islamic State claimed responsibility for the shooting in a statement on Amaq, its propaganda news agency. The claim has puzzled the investigators and extremism experts alike.

Investigators found no evidence that Paddock had been in contact with other individuals or groups about the attack. They found no proof that he had become radicalized, supported or followed terrorist organizations, or even adhered to a particular ideology. Nothing indicates that Paddock ever converted to Islam, and ISIS has never released any proof of its involvement.

None of the indicators in other ISIS cases in the U.S. apply to this guy as far as we know, said Amarnath Amarasingam, a senior research fellow at the Institute for Strategic Dialogue who closely followed the groups actions related to the attack.