MGM Resorts International said it has identified a cyberattack that is affecting some of the company’s systems.

The Las Vegas-based company said it has notified law enforcement and began an investigation with the help of external cybersecurity experts. The company also said it took “prompt action to protect our systems and data, including shutting down certain systems.”

A representative couldn’t immediately be located for comment, but previously told the Associated Press that the incident began Sunday.

The extent of the hack wasn’t immediately known. A message on MGM’s website, which lists numbers for concierges at 19 hotels across the country, says the site “is currently unavailable.”