At least two persons have been reported dead and 24 others injured after
Police told journalists that "one suspect is down".
About 12 people are also said to be in critical condition.
The three-day country music concert, which reportedly attracted 30,000 attendees, held over the weekend.
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Reports say the sound of gunshots rented the air as Jason Aldean, one of the final performers, was playing.
The Associated Press cited witnesses who said they could see muzzle flashes coming from the Mandalay Bay Hotel and Casino's high floors and described the sound of automatic gunfire.
Police have since cordoned off the area while flights from the city's airport have been suspended and part of the strip closed off.