Lifeboat Foundation

Not cool, China.

(Reuters) — Hackers behind a massive breach at hotel group Marriott International left clues suggesting they were working for a Chinese government intelligence gathering operation, according to sources familiar with the matter.

Marriott said last week that a hack that began four years ago had exposed the records of up to 500 million customers in its Starwood hotels reservation system.

Private investigators looking into the breach have found hacking tools, techniques and procedures previously used in attacks attributed to Chinese hackers, said three sources who were not authorized to discuss the company’s private probe into the attack.