NEW YORK, U.S. - Days after the major hotel group, Marriott International Inc revealed that it had suffered a massive cyberattack that compromised data of about 500 million customers, sources have revealed further details of the hacking.

On December 1, Marriott International revealed that hackers stole about 500 million records from its Starwood Hotels reservation system.

The star hotel, which was dubbed to have suffered one of the largest hacking attacks in history, said that the attack began four years back.

It said the cyberattack started a year before Marriott began its $13.6 billion-worth acquisition of Starwood Hotels in November 2015, which led to the creation of the world's largest hotel operator. 

According to Marriott's statement, off the 500 million guests impacted in the data breach, 327 million guests were found to have had their passport details, birthdates, addresses, phone numbers and email addresses exposed, while payment card data for an undisclosed number of customers too was accessed by hackers.

Marriott said that it was made aware of the breach on September 8 after an internal security tool sent an alert about suspicious activity and added that it was informing affected guests.

Who is behind the hack?

While the revelation by Marriott prompted regulators in Britain and at least five U.S. states to launch investigations, now, sources quoted in a Reuters report have revealed possible suspects in the case. 

According to sources familiar with the matter, hackers behind the massive breach left clues suggesting they were working for a Chinese government intelligence gathering operation.

The report stated that private investigators examining the breach had found hacking tools, techniques and procedures that have previously been used in attacks attributed to Chinese hackers.

The information came from three sources who spoke about the private probe into the attack anonymously.

They said that the clues available to investigation currently suggest that Chinese hackers may have been behind the campaign, which was reportedly designed to collect information for use in Beijing's espionage efforts.

The hacking was not believed to have been aimed at making financial gains.

While revealing the suspicions hinting towards the high-level Chinese role, sources also warned that it was possible somebody else was behind the hack since the hacking tools believed to have been used in the attack were previously posted online and other parties too had gained access to these tools.

The sources pointed out that investigators are pursuing other lines of inquiry, which point at the possibility of involvement of multiple hacking groups, who could have simultaneously been inside Starwood's computer networks since 2014.

This, the source said, was making the identification of culprits much more complicated.

Following the disclosure by sources, Marriott refused to comment and the hotel spokeswoman Connie Kim said, "We've got nothing to share."

However, experts fear further complication in ties between the U.S. and China, if investigations confirm Beijing's suspected role in the cyberattack.

Washington and Beijing are involved in an ongoing tariff dispute that has lasted months now and has resulted in economic damages on both sides.

Further, ties between the world's two largest economies have also suffered due to U.S. accusations of Chinese espionage and the theft of trade secrets.