Chinese hackers are suspected of infiltrating the US government’s Office of Personnel Management (OPM) and making away with sensitive data on roughly four million current and former federal employees.

In a statement to The Wall Street Journal, the OPM said it first detected the breach in April and reached out to the Federal Bureau of Investigation and the Department of Homeland Security. It wasn’t until early May, however, that officials realized data had been compromised.

One official told the publication it is one of the largest thefts of government data ever seen.

OPM Director Katherine Archuleta said the agency takes very seriously their responsibility to secure the information stored in their systems and in coordination with agency partners, their experienced team is constantly identifying opportunities to further protect the data with which they are entrusted.

The four million figure is just an early estimate. The OPM added that it’s possible that even more records were swiped. The ratio of government officials versus contractors that were exposed wasn’t yet known, the firm said.

Chinese hackers have been suspected of multiple security breaches in recent memory. Earlier this year, security experts pegged China as the likely culprit behind a 2014 cyber attack against Forbes. The US Postal Service also fell victim to an attack in which personal data on 800,000 employees was compromised. Chinese government hackers were named as the prime suspect in that investigation.

The Chinese embassy in Washington didn’t respond to a request for comment as of the Journal’s deadline.