A crime with no obvious explanation. No telltale signs. But today, sources say some things are finally starting to make sense.
The mood at the FBI is being described as "upbeat" as pieces are starting to come together.
"What we know is Stephen Paddock is a man who spent decades acquiring weapons and ammo and living a secret life, much of which will never be fully understood," Las Vegas Metropolitan Police Department Sheriff Joe Lombardo said in a press conference on Wednesday.
What authorities do understand is that Paddock comes from a family history of mental illness. Paddock's estranged father, Benjamin Hoskins Paddock, was a notorious bank robber who spent years as one of the country's most wanted fugitives.
A wanted poster circulated in 1969 described Paddock as "diagnosed as psychopathic... He carried firearms in commission of bank robberies. He reportedly has suicidal tendencies and should be considered armed and very dangerous."
"More than 100 investigators have spent the last 72 hours combing through the life 64-year-old Stephen Paddock to produce a profile of someone I will call disturbed and dangerous," Lombardo said.
Mental illness can be genetic and sources say Paddock's descent into madness seems to have started around the same time in life as his father.
He's also been described as highly intelligent.
"He meticulously planned on the worst domestic attack in United States history," said Sheriff Lombardo.
Part of that planning may have included renting units during music festivals across the country, including at the Ogden downtown during Life is Beautiful just one week before the shooting.
"Reasons that ran through Paddock's mind is unknown," Lombardo said. "But it was directly during the same time as Life is Beautiful."
The unit was rented through Airbnb. They confirm that once they heard about the shooting early on Monday, they researched Paddock's reservation history, found the September booking at the Ogden and immediately turned that and the message traffic between Paddock and his host over to Las Vegas Metropolitan Police and the FBI.
The message history contains a discussion about whether Paddock would be able to see the concert from the unit.
Airbnb says that was Paddock's only reservation history with them, adding that they did a background check on him as they do with all hosts and renters and it came back clean.
"Was he doing pre-surveillance? We don't know yet," Lombardo said Wednesday.
ABC News also found Paddock booked rooms in Chicago in the first week of August, during the Lollapalooza festival in Grant Park.
Investigators don't know if he actually traveled to Chicago but are working now to find out.
Paddock also searched for hotels near Fenway Park in Boston, but there is no indication he traveled there.
As for the latest on the investigation into whether Paddock acted alone, officials believe he was seen with another woman - who is not his girlfriend Marilou Danley - in his final days.
Her identification is not known and they are working to find out who she is and if she knows anything.