Authorities in Las Vegas have clashed over whether Stephen Paddock acted alone when he shot at a music festival.
Police in Las Vegas have suggested that Paddock may have had outside help but the FBI have slapped down such claims.
Asked at a press conference if Paddock acted alone, Sheriff Joseph Lombardo said: "You've got to make the assumption he had to have some help at some point."
But Lombardo was rebuked by the FBI who disagreed with the idea that others were involved in the attack.
Aaron Rouse who runs the Las Vegas division of the FBI said: "Theories are great and everyone can have a theory.
"But I need to deal with facts. The sheriff needs to deal with facts. He's not going to make assumptions. I'm not going to make assumptions."
Lombardo argued that he "found it hard to believe" that he was able to carry out such a sophisticated attack unless he was a "super guy."
Police also revealed that former accountant Paddock had reserved a room overlooking another festival the previous week in the city but authorities were unsure what his purpose for the booking was.
A total of 58 people were killed in the attack and a further 500 were injured in the deadliest mass shooting in living US history.
The attack took place on Sunday evening when 64-year-old Paddock, positioned in a room on the 32nd floor of the Mandalay Bay Hotel with a cache of high-powered weapons , opened fire on a crowd of 23,000 who were watching the closing moments of a three-day country music festival.
Paddock was found dead in his hotel with a self-inflicted gunshot.
On Wednesday 4 October, Donald Trump visited Las Vegas, praising the work of emergency services and condemning Paddock's act.