Authorities said Paddock, 64, fired from the 32nd floor of the Mandalay Bay complex into a crowd at a country music concert on the Las Vegas Strip Sunday night. At least 50 people were killed and 400 more were taken to the hospital.
As police entered the room, Paddock killed himself, authorities said.
PHOTOS: Stephen Paddock
Eric Hudson Paddock, of Orlando, Fla., told NBC that he had "no idea" why his brother committed the shooting.
“Mars just fell into the earth,” he told NBC News. “We’re completely dumbfounded.”
Paddock said his brother was retired and was "just a guy" who went to the hotels, gambled, and went to shows.
“We are completely at a loss,” Eric Paddock said.
Eric Paddock said his brother was never an "avid gun guy" and didn't have any background in the military.
According to NBC, Paddock was a licensed pilot who owned two planes. He had a hunting license from Alaska, NBC said. He lived in a retirement community in Mesquite, Nev., according to NBC.
Police said that so far, they believe Paddock acted as a "lone wolf."
Authorities said they found 23 firearms in the Mandalay Bay hotel room and 19 firearms in the suspects home.
Material from USA Today was used in this report. Catherine Park, an Associate Digital Producer with 11Alive News contributed to this report.