The cybercrime group has made a ransom demand to MGM as well, those sources told CNBC's Contessa Brewer. The 8-K report, similar to one filed by MGM Resorts on Wednesday, acknowledges the hack as a material event. The cybercrime group demanded a $30 million ransom from Caesars, but the company ultimately agreed to pay about half that, sources said. Bloomberg previously reported the ransom and that the same group is behind the attacks on both companies. It wasn't immediately clear why Caesars delayed filing the report disclosing the hack and ransom for weeks.

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Persons: CNBC's Contessa Brewer, Charles Carmakal Organizations: Caesars, CNBC, MGM, U.S . Securities, Exchange, MGM Resorts, Google, Bloomberg, vx, Twitter, Security, SEC Locations: United States, Twilio