Country-Western concert at outdoors site across the street from Mandalay Bay casino and hotel. Attendance 22,000 to 40,000 people (depending on what source you believe). Toward the end of the show, someone started shooting into the concert audience with what sounded like a machine gun. Shots were noted to be coming from an upper floor of the Mandalay Bay hotel. People scrammed. Many people shot, many others injured in the stampede.
Police responded. Shooter traced to a room on 32nd floor thanks to the gunsmoke setting off the smoke alarm in his room. Police breached door to room, shooter killed himself. Shooter reported to have 10 AR-15s and 5,000 rounds of ammunition. Reportedly, multiple rifles were equipped with slide-fire stocks.
Shooter was a retired accountant and high stakes gambler. Political and religious affiliations unknown -- shooter's brother says there were none that the family knows of.
Motive unknown. Police searching his home. His female roommate (nature of relationship unknown) is out of the country and is not considered a suspect.