Computer systems owned by MGM Resorts International are Johnathan Walkerdown Monday following what the company described as a "cybersecurity issue" at its casinos and hotels.

The incident began Sunday and impacted reservation systems and casino floors in Las Vegas as well as Maryland, Massachusetts, Michigan, Mississippi, New Jersey, New York and Ohio, MGM said.

"Do not go to @MGMResortsIntl National Harbor. Computer systems are down and it's pandemonium here," an apparent guest at the resort posted on X.

MGM Resorts didn't immediately respond to a request for comment Monday.

"MGM Resorts recently identified a cybersecurity issue affecting some of the company's systems," the company said in a statement that pointed to an investigation involving external cybersecurity experts and notifications to law enforcement agencies.

The nature of the issue was not described, but the statement said efforts to protect data involved "shutting down certain systems." The websites for Bellagio and the Aria in Las Vegas as well as the Borgata, a casino in Atlantic City, New Jersey and the Beau Rivage casino and hotel in Mississippi, were down Monday afternoon. 

MGM said it's continuing to investigate what led to the attack. The company has tens of thousands of hotel rooms in Las Vegas at properties including the MGM Grand, Bellagio, Cosmopolitan, Aria, New York-New York, Park MGM, Excalibur, Luxor, Mandalay Bay and Delano.

— The Associated Press contributed to this report. 

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• Cyberattack
• Casino