WASHINGTON (TIP): Hackers broke into US government computers, possibly compromising the personal data of 4 million current and former federal employees, and investigators were probing whether the culprits were based in China, US officials said on June 4.

In the latest in a string of intrusions into US agencies’ high-tech systems, the Office of Personnel Management suffered what appeared to be one of the largest breaches of information ever on government workers. The office handles employee records and security clearances.

A US law enforcement source told Reuters a “foreign entity or government” was believed to be behind the cyber attack. Authorities were looking into a possible Chinese connection, a source close to the matter said.

The Federal Bureau of Investigation said it had launched a probe and aimed to bring to account those responsible.

OPM detected new malicious activity affecting its information systems in April and the Department of Homeland Security said it concluded at the beginning of May that the agency’s data had been compromised and about 4 million workers may have been affected.

The agencies involved did not specify exactly what kind of information was accessed.

The breach hit OPM’s IT systems and its data stored at the Department of the Interior’s data center, a shared service center for federal agencies, a DHS official said on condition of anonymity. The official would not comment on whether other agencies’ data had been affected.