US officials suspect hackers in China are responsible for a cyber attack on the Office of Personnel Management’s (OPM) computer systems that left the personal information of almost four million current and former federal government employees exposed.

The breach, which was detected back in April, is now being described as one of the largest thefts of government data ever seen. DHS concluded back in May that the information had indeed been compromised and stolen, but so far neither OPM of the FBI have indicated exactly whose records have been exposed.

More from WaPo:

“Certainly, OPM is a high value target,” said OPM Chief Information Officer Donna Seymour, in an interview. “We have a lot of information about people, and that is something that our adversaries want.”

With that understanding, she said, within the last year “OPM has undertaken an aggressive effort to update our cybersecurity posture, adding numerous tools and capabilities to our networks. As a result of adding these tools, we were able to detect this intrusion into our networks.”

“Protecting our federal employee data from malicious cyber incidents is of the highest priority at OPM,” said the agency’s director, Katherine Archuleta, in a statement.

According to an unnamed Congressional aide, the Interior Department was also hacked. Another source for CBS News said that the data breach could potentially affect every government agency.

If the hack is traced back to China, it will be the second major breach executed by Chinese hackers in the space of a year, and one of several hacks affecting federal workers:

In December 2014, the government confirmed that the computer files of more than 40,000 federal workers may have been compromised by a cyberattack at federal contractor KeyPoint Government Solutions. KeyPoint became the largest private clearance firm working for federal agencies several months ago after rival contractor United States Investigative Services (USIS) lost its investigations business with the government following a devastating cyberattack reported earlier that year.

The USIS breach, similar to previous hacking episodes traced to China, tainted the files of at least 25,000 Department of Homeland Security workers and prompted the personnel office’s decision to halt all of USIS’ government field work. That move led to the cancellation of more than $300 million in contracts with USIS.

A wide-ranging strike reported in November compromised the data of more than 800,000 Postal Service workers. The personnel office itself was targeted earlier by cyberhackers traced to China.

We’ll keep you updated on the investigation.