The U.S. government’s Office of Personnel Management (OPM) is the latest victim of a coordinated hack attack, and the Chinese are again suspected of being behind it.

The attackers stole the personal data of as many as 4 million people, a Wall Street Journal report said. But the OPM said the losses could be even more.

The OPM said in a statement that it discovered the breach in April, and became sure that the records had indeed been stolen in early May.

The FBI and the Department of Homeland Security are investigating.

“We take very seriously our responsibility to secure the information stored in our systems, and in coordination with our agency partners, our experienced team is constantly identifying opportunities to further protect the data with which we are entrusted,” OPM director Katherine Archuleta told the WSJ.

The Chinese embassy had no comment, the WSJ said.

Just last week hackers accessed the servers at the Internal Revenue Service and stole data from as many 100,000 taxpayers.The thieves accessed the data through an online service provided by the IRS and took financial data related to users’ actual tax returns.