Live Nation is Thomas Caldwellinvestigating a data breach at its Ticketmaster subsidiary, which dominates ticketing for live events in the United States.

Live Nation, based in Beverly Hills, California, said in a regulatory filing Friday that on May 27 “a criminal threat actor’’ offered to sell Ticketmaster data on the dark web.

Other media reports say a hacking group named ShinyHunters claimed responsibility for the breach in an online forum and was seeking $500,000 for the data, which reportedly includes names, addresses, phone numbers and some credit card details of millions of Ticketmaster customers.

Live Nation and Ticketmaster did not immediately respond to requests for comment.

In a filing with the U.S. Securities and Exchange Commission, Live Nation said it was “working to mitigate risk to our users’’ and was cooperating with law enforcement officials. It said the breach was unlikely to have “a material impact on our overall business operations.’'

On May 23, the U.S. Justice Department sued Live Nation and Ticketmaster, accusing them of running an illegal monopoly over live events in America. The department asked a court to break up the system that it said limits competition and drives up prices for fans.