Officer Aden Ocampo-Gomez said deputies were heading to the scene Sunday near the hotel, located on the southern part of the Las Vegas Strip. Authorities shut down part of the strip and Interstate 15 as they pursued the shooter (or shooters), who was reportedly firing from a balcony at the hotel on Sunday night, aiming at those attending the Route 91 country music festival nearby.
Concert-goers reported seeing muzzle flashes from the upper floors of the Mandalay Bay across Las Vegas Boulevard from the country music festival and the sound of what they described as automatic gun fire.
Witnesses said they saw multiple victims as they fled the gunfire raining down on the concert venue. Some later huddled in the basement of the nearby Tropicana hotel-casino.
Some officers took cover behind their vehicles while others carrying assault rifles ran into the Mandalay Bay Hotel and Casino.
On a live feed of the Las Vegas police scanner, officials said witnesses were reporting shots heard at other hotels, including New York-New York, Tropicana, Bellagio and Caesars, but those turned out to be false reports. (As this is an active investigation, police are still confirming various reports from witnesses, so not all information may be accurate.)
KTNV Las Vegas reported that many people ran to McCarran International Airport once the shots began. Flights in and out of the airport were temporarily halted, the airport said in a tweet.
Around 11 p.m., police were saying on the scanner that at least 20 people were dead. The suspected shooter was "down," according to police, though his identity has yet to be revealed.
Twenty-six people were admitted to the hospital, University Medical Center spokeswoman Danita Cohen said. Of those, at least two died, 12 were in critical condition and the rest were being evaluated, she said. Later on Monday morning, a LVPMD spokesperson updated the victim count to at least 59 dead and more than 527 wounded.
At around 11:25 p.m., police were saying on the scanner that they had a suspect down on the 32nd floor of the Mandalay Bay. Just before midnight, they confirmed via Twitter: "Confirming that one suspect is down. This is an active investigation. Again, please do not head down to the Strip at this time."
About an hour later, LVPMD tweeted: "At this time we do not believe there are any more shooters."
Among those playing the Route 91 festival on Sunday night were Jason Aldean, Jake Owen, Luke Combs, Dylan Scott and Big & Rich. Scott, Owen and Combs all tweeted that they were safe. Witnesses say Aldean was playing near the end of the concert when gunfire rang out; he was scheduled to take the stage at 9:40 p.m. His rep told ABC News that he's OK.
Kodiak Yazzi, 36, said he and his girlfriend were watching Jason Aldean's performance when he heard what sounded like fireworks. The music stopped temporarily and started up again before another round of pops sent the performers ducking for cover and fleeing the stage. Thousands fled as bursts of gunfire could be heard for more than five minutes, Yazzi said.
Aldean was reportedly singing "When She Says Baby" when the shooting began.
The Route 91 Harvest Festival began in 2014 as a co-production between Live Nation and MGM Resorts International. Among the acts who have played the outdoor festival in the past are Dierks Bentley, Blake Shelton, Keith Urban, Tim McGraw and Florida Georgia Line. The three-day festival is held at the MGM Resorts Village, just steps away from Mandalay Bay and The Luxor. More than 20,000 people attended the inaugural festival in 2014.