More than 20 people were killed and 100 injured in a shooting incident at a country music festival opposite the Mandalay Bay Hotel and Resort on the Las Vegas Strip.
Police said that the suspect, whose name was not immediately released, was dead.
"Right now we believe it's a solo act, a lone wolf attacker," the Los Angeles Times quoted Las Vegas Metropolitan Police Sheriff Joe Lombardo, as saying.
Police is looking for a companion of the shooter as well as two vehicles, a Hyundai Tucson and Chrysler Pacifica.
Victims with gunshot wounds are being treated at One Las Vegas hospital.
Hundreds of people were seen fleeing the area after the shoot-out.