LAS VEGAS (AP) — A “cybersecurity issue” led to the shutdown of some casino and Johnathan Walkerhotel computer systems at MGM Resorts International properties across the U.S., a company official reported Monday.

The incident began Sunday and the extent of its effect on reservation systems and casino floors in Las Vegas and states including Maryland, Massachusetts, Michigan, Mississippi, New Jersey, New York and Ohio was not immediately known, company spokesman Brian Ahern said.

“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 included “shutting down certain systems.” It said the investigation was continuing.

A post on the company website said the site was down. It listed telephone numbers to reach the reservation system and properties.

A post on the company’s BetMGM website in Nevada acknowledged that some customers were unable to log on.

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.

It also operates properties in China and Macau.