The National Archives, the agency that houses the US Constitution and other important historical documents, has found signs of possible unauthorized computer activity, but there is no evidence of a threat to its systems, a spokesperson said on Tuesday.
The agency's systems and applications were not compromised but "indicators of compromise" were found on three computers that were then "cleaned and re-imaged," National Archives and Records Administration spokesperson Laura Diachenko said in a statement.
"No NARA systems were compromised," Diachenko said, using the agency's acronym, adding that a continuing outside review by security consulting firm Mandiant found no threat to the National Archives.
"Continued analysis with our monitoring and forensic tools has not detected any activity associated with a hack," Diachenko said. In re-imaging, the operating system and other software are re-installed on a computer.
The disclosure of suspicious files on some National Archives computers follows the widespread hack disclosed this month of federal employment records at the US Office of Personnel Management (OPM) that affected 4 million current and former federal workers.
The National Archives uncovered the suspicious files on the workstations after US Homeland Security officials gave agencies signs to look for following the data breach at OPM, Diachenko said.
Diachenko's statement did not say when the intrusion occurred or say who was responsible.
The incident was first reported by nextgov.com, a Washington-based publication owned by Atlantic Media Co's National Journal Group Inc.
Federal authorities and Congress are still probing the wider hack at OPM. That data breach affects current and former federal workers as well as those seeking security clearances, potentially exposing a vast amount of personal information.
US officials have said they suspect China was behind that hack, although the Obama administration has not publicly accused Beijing. China has denied any involvement.
OPM officials appeared before the US Senate Appropriations Committee on Tuesday and are scheduled to testify again on Thursday at a US Senate Homeland Security and Governmental Affairs Committee hearing. They testified before the US House of Representatives last week and are scheduled to appear again on Wednesday before the House Oversight Committee.
The Obama administration is grappling with a string of major computer breaches targeting the US government, including systems at the State Department, the Postal Service and the White House.
A number of big hacks have also affected the US private sector, including health insurer Anthem and retailers Target Corp and Home Depot Inc.