Chinese hackers are suspected of breaking into U.S. government computers last year.
Reuters sources say that the hackers exploited a flaw in software made by SolarWinds to target a federal payroll agency.
SolarWinds is the company that alleged Russian hackers used to breach U.S. government networks and private firms last year.
That attack only came to light in December, but hackers are believed to have breached the networks for months.
U.S. lawmakers have called the alleged Russian hack a national security emergency, but this latest breach by suspected Chinese hackers marks a new twist in the saga.
The Chinese hackers are believed to have exploited a different flaw in the software of SolarWinds to the Russian hackers.
According to sources, FBI investigators recently found that the National Finance Center was affected by the breach.
It's responsible for handling the payroll of several federal agencies, including the Department of Homeland Security.
Reuters was not able to establish how many organizations were affected by the suspected Chinese operation.
But there are fears that data on thousands of government employees may have been compromised.
The FBI declined to comment.
China's foreign ministry said any allegations should be supported with evidence.
Reuters sources say the two espionage efforts were separate and distinctly different operations.
But both show how hackers are now focusing on weaknesses in obscure but essential software products that are widely used by major corporations and government agencies.