https://www.cnet.com/news/chinese-spies-reportedly-behind-massive-marriott-hack/

Private investigators involved in a probe into the breach had previously discovered hacking tools, techniques and procedures that were used in earlier cyberattacks that have been linked to Chinese hackers. 

A Marriott spokeswoman said it had "no information about the cause of this incident" and hadn't speculated about the attacker's identity.