The Las Vegas gunman, who killed concertgoers in the city last weekend leaving 59 people dead and more than 500 people wounded, had first opened fire on a security guard.
The guard was investigating an alarm signalling that a door had been left open on the same floor as Paddock's room, officials said on Monday.
However, the authorities do not know what made him stop shooting, as yet, the CNN reported.
Stephen Paddock, the Nevada man who authorities have identified as the gunman, "continued to fire at a progressive successive rate for approximately nine minutes" after authorities received a 911 call, Clark County Sheriff Joseph Lombardo said.
It took about 10 minutes of gunfire for Stephen Paddock to carry out the deadliest mass shooting.
The investigators found that Paddock meticulously planned the shooting. He checked into a hotel room overlooking the music festival, stocked a cache of weapons there and set up cameras inside his hotel suite and hallway.
They recovered 42 firearms, an assortment of explosives and several thousand rounds of ammunition from his Las Vegas hotel room and his Mesquite home.