The company said that its sites in Las Vegas, Maryland, Massachusetts, Michigan, Mississippi, New Jersey, New York, and Ohio may have been impacted by the problem.

MGM Resorts International said Monday that it had a “cybersecurity issue” that could have affected its hotels, casinos, and entertainment venues all over the U.S.

The problem may still be affecting the company that is open to the public: Monday night, some of its websites were down, so it told people to call to book rooms and make plans.

The full effect on ticket systems and gaming halls in Las Vegas, where the company is based, as well as in Maryland, Massachusetts, Michigan, Mississippi, New Jersey, New York, and Ohio was not known, said spokesman Brian Ahern.

In a statement released Monday night, the company said that the problem was still going on but that its casinos were still open for business. “We are still working hard to solve this problem,” it said.

MGM resorts said earlier in the day that “some of the company’s systems” were damaged and that the police had been told.

Some MGM systems were shut down to protect data, and the company started an internal review with the help of “leading external cybersecurity experts,” according to the statement.

When asked for comments, the FBI in Las Vegas and the Nevada Gaming Control Board did not reply.

MGM lists 19 locations in the United States. Some of the most popular Las Vegas casinos, like the Bellagio, Mandalay Bay, and Cosmopolitan, are on this list. It also owns land in China.

Late last year, Nevada’s gaming board passed tougher steps to protect online systems, such as giving people three days to report any problems.

In July, the Securities and Exchange Commission put in place a similar rule for big companies that are sold on the stock market. It says that a major breach must be reported within four business days, but this won’t be a rule until December.