By Andrea Shalal and Matt Spetalnick
WASHINGTON (Reuters) - Data stolen from U.S. government computers by suspected Chinese hackers included security clearance information and background checks dating back three decades, U.S. officials said on Friday, underlining the scope of one of the largest known cyber attacks on federal networks.
The breach of computer systems of the Office of Personnel Management was disclosed on Thursday by the Obama administration, which said records of up to 4 million current and former federal employees may have been compromised.
U.S. government sources said the hackers were believed to have been based in China but it was not yet known if the massive hacking was state-sponsored.
The breach was among the most far-reaching thefts of information on the federal work force.
"This is deep. The data goes back to 1985," said one official, who spoke on condition of anonymity. "This means that they potentially have information about retirees, and they could know what they did after leaving government."
Access to information from OPM's computers, such as birthdates, Social Security numbers and bank information, could help hackers test potential passwords to other sites, including those containing information about critical weapons systems, the official said.
"That could give them a huge advantage," the official said.
Cyber investigators have linked the OPM breach to earlier thefts of personal data from millions of records at Anthem Inc, the second largest U.S. health insurer, and Premera Blue Cross, a healthcare services provider.
It was the second computer break-in in less than a year at OPM, the federal government's personnel office and the latest in a string of cyber attacks on U.S. agencies, some of which have been blamed on Chinese hackers.
A Chinese Foreign Ministry spokesman said such accusations had been frequent of late and were irresponsible. Hacking attacks were often cross-border and hard to trace, he said.
But another U.S. official said the United States believed the hack was of “Chinese origin,” but the source offered no details on how U.S authorities came to that conclusion.
Disclosure of the latest computer breach comes ahead of the annual U.S.-China Strategic and Economic Dialogue due to be held in Washington on June 22-24. Cyber security was already expected to be high on the agenda.
Tension between Washington and Beijing has intensified over Chinese assertiveness in maritime disputes with its neighbors in the South China Sea.
The Federal Bureau of Investigation said it aimed to bring to account those responsible for the hacking.
OPM detected new malicious activity affecting its information systems in April and the Department of Homeland Security (DHS) said it concluded at the beginning of May that the agency's data had been compromised and about 4 million workers may have been affected.
The breach hit OPM's IT systems and its data stored at the Department of the Interior's data center, a shared service center for federal agencies, a DHS official said on condition of anonymity.
Chinese hackers were blamed for penetrating OPM's computer networks last year, and hackers appeared to have targeted files on tens of thousands of employees who had applied for top-secret security clearances, the New York Times reported last July, citing unnamed U.S. officials.
(Additional reporting by Phil Stewart and David Brunnstrom; Editing by Jason Szep, Doina Chiacu, Toni Reinhold)