The FBI is investigating a hack into the network of the Federal Deposit Insurance Corporation (FDIC), which is said to have lasted years, according to a Reuters report.
Officials at the federal agency, which regulates commercial banks in the United States, are pointing fingers at China's military for the incursion, though no evidence has been presented.
Dozens of FDIC computers, including one belonging to former FDIC chairwoman Sheila Bair, were impacted by the attack, believed to have started in 2010.
The agency, which manages strategies big banks would take to handle a bankruptcy, has access to data on millions of individual American deposits.
Internal communications related to the breach were examined last month by congressional staff. It has not been determined how long the FBI has been investigating the case, though the probe was described as still active.
The FDIC has informed Congress of at least seven cyber-security incidents this year. Additionally, under a Freedom of Information Act request, Reuters obtained a redacted copy of an annual report from the regulator that reported that computers were accessed by unauthorised individuals 159 times during fiscal year 2015.
Many of those incidents were described as security lapses, such as an employee copying sensitive data to a thumb drive, although, 20 were confirmed as data breaches, according to an FDIC document provided to Reuters by the US House of Representatives Committee on Science, Space and Technology.
A report in July from the committee named China as the actors behind incursions into FDIC computers, though at that point a link to the nation's military was not mentioned.
In September, officials at the FDIC informed the congressional committee it could not provide requested documents as the FBI was investigating the attacks.
The FDIC claimed it is tightening security measures – such as banning thumb drives and firming up partnerships with the Department of Homeland Security to better defend against hacks.