(TIML NEWS) Police investigated an active shooter situation in Las Vegas at the Mandalay Bay Hotel and Casino on Sunday night that continued into the early hours of Monday morning.

Dozens of patrol vehicles descended on the Strip after authorities received reports of an active shooter near the Route 91 Harvest Festival.

Around 1:30 a.m. local time Monday, police said the gunman was dead and said he had killed more than 20 people, with 100-plus injured, but by 3:45 a.m the death toll rose to more than 50 and more than 400 injured. “At this time there is in excess of 50 deceased and over 400 injured individuals,” police tweeted at the time.

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.)