Investigators were "very confident that there was not another shooter in that room", McMahill said. "We have been down each and every one of these paths", McMahill said. "We all want answers".
The billboards will bear the slogan, "If you know something, say something", and carry a toll-free number to an FBI hotline, said Aaron Rouse, special agent in charge of the Las Vegas FBI office. Investigators have reviewed video and found no indication that there is another suspect.
Sunday's attack was in stark contrast to recent mass shootings, which have had motives surface soon afterward. Previous incidents were made clear by a social media post, note, phone call, or through computer evidence. Turns out, he did leave behind a note of some sort, but it wasn't a suicide note.
Tannerite was previously found in a auto Paddock parked at the Mandalay Bay and in another of his homes, in Mesquite, Nev.
Termed as one of the deadliest mass shootings in modern United States history, the Las Vegas attack opened eyes to the contradicting views people have adopted against Muslims.
The FBI also announced a new information campaign seeking more details into the shooting. Paddock fired at a security guard on Sunday night and, at some point before a SWAT team entered his suite, killed himself.
Paddock bought other ammunition at the show, but he couldn't obtain the tracer ammunition because the vendor didn't have any to sell, the official said.
The officers were the first to see the body of Stephen Paddock, who had taken his own life, and an arsenal of at least 23 semi-automatic rifles - 12 of which were equipped with bump-stocks, or rapid fire devices - found inside his hotel room.
Gunman Stephen Paddock's personal life, mental health, political affiliation and finances are being scrutinised but have not yet offered any clues, Undersheriff Kevin McMahill said.
His longtime girlfriend Marilou Danley said Paddock "would lie in bed, just moaning and screaming, 'Oh my God, '" according to an NBC News report.
Investigators are still probing whether anyone else might have had knowledge of Paddock's plans before the shooting, McMahill said. She was out of the country at the time of the attacks and has been labeled a "person of interest", though she's not in custody and is cooperating with authorities.
The few people who did know Paddock said they had no hint he was capable of such an act, let alone that he was actively planning it.
Rukmini Callimachi, a reporter with The New York Times who covers the fanatics for the influential newspaper, said IS's claim carried weight.