WASHINGTON (AP) -- Hackers stole Social Security numbers, health histories and other highly sensitive data from more than 21 million people, the Obama administration said Thursday, acknowledging that the breach of U.S. government computer systems was far more severe than previously disclosed.

Computer used by a Florida cyber crimes unit (Joe Raedle/Getty Images)

The scope of the data breach - believed to be the biggest in U.S. history - has grown dramatically since the government first disclosed earlier this year that hackers had gotten into the Office of Personnel Management's personnel database and stolen records for about 4.2 million people. Since then, the Obama administration has acknowledged a second, related breach of the systems housing private data that individuals submit during background investigations to obtain security clearances.

That second breach affected more than 19 million people who applied for clearances, as well as nearly 2 million of their spouses, housemates and others who never applied for security clearances, the administration said. Among the data the hackers stole: criminal, financial, health, employment and residency histories, as well as information about their families and acquaintances.

The new revelations drew indignation from members of Congress who have said the administration has not done enough to protect personal data in their systems, as well as calls for OPM Director Katherine Archuleta and her top deputies to resign. House Oversight and Government Reform Committee Chairman Jason Chaffetz, a Utah Republican, said Archuleta and her aides had "consciously ignored the warnings and failed to correct these weaknesses."

"Such incompetence is inexcusable," Chaffetz said in a statement.

Yet Archuleta insisted she would not step down. "I am committed to the work that I am doing," she said in a conference call with reporters.

Archuleta said the hackers also got hold of the user names and passwords that prospective employees used to fill out their background investigation forms, as well as the contents of interviews conducted as part of those investigations. Yet the government insisted there were no indications that the hackers have used the data they stole.

Still, the government declined to say who was behind the attack.

Numerous U.S. lawmakers, including Senate Democratic leader Harry Reid, have said China was behind the attack. But Michael Daniel, President Barack Obama's cybersecurity coordinator, said the government wasn't yet ready to say who was responsible.

"Just because we're not doing public attribution does not mean that we're not taking steps to deal with the matter," Daniel told reporters.

While officials would not point the finger at China, they acknowledged that the same party was responsible for both of the breaches, which took place in 2014 and early 2015. Investigators previously told The Associated Press that the U.S. government was increasingly confident that China's government, and not criminal hackers, was responsible for the extraordinary theft of personal information.

China has publicly denied involvement in the break-in.

Earlier Thursday, during a round-table discussion with reporters, FBI Director James Comey described the scope of the OPM breach as "huge" and called it "a very big deal from a national-security perspective and a counterintelligence perspective."

"It's a treasure trove of information about everybody who has worked for, tried to work for, or works for the United States government," he said.

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