Washington (AFP) - Hackers who breached US government databases stole personal information from background checks of 21.5 million people, officials said Thursday following an investigation into the massive attack widely blamed on China.

An update from the government's Office of Personnel Management said those affected were 19.7 million who underwent a background investigation, and 1.8 million others, mostly spouses or cohabitants of applicants for government jobs.

The massive figure adds to the gravity of the breach, which prompted a series of hearings in Congress and widespread criticism of the state of US cyber-defenses.

Officials said last month 4.2 million personnel records were breached in a separate attack affecting current, former and prospective federal employees.

OPM director Katherine Archuleta said 3.6 million of those whose background checks were stolen were also in the group whose personnel records were breached, thus bringing the number of individuals affected in total to 22.1 million.

The investigation relates to "the second of two separate but related cybersecurity incidents" affecting federal databases, Archuleta told reporters in a conference call.

She said that in addition to sensitive social security numbers, the attackers likely obtained data on health, financial, criminal and family history of people who sought government jobs requiring a security clearance. Some 1.1 million fingerprints were also stolen.

An OPM statement noted that for anyone who underwent a background investigation in 2000 or afterwards "it is highly likely that the individual is impacted by this cyber breach."

Archuleta said there was "no information to suggest any misuse" of the data, but that the government would be offering free monitoring to those affected to guard against fraud or identity theft.

- No comment on attackers -

Officials in the call declined to comment on the assertion that China was behind the massive breach, even though intelligence chief James Clapper said last month that Beijing was "the leading suspect."

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Michael Daniel, cybersecurity coordinator at the White House National Security Council, said that "the investigation into the attribution of this is still ongoing... and we're not yet prepared to comment" on who was behind the attacks.

But Daniel added that "just because we are not doing public attribution does not mean we are not taking steps to deal with this."

Andy Ozment, an assistant secretary of Homeland Security, said however that the attacks came from "the same actor, moving between different networks."

He said the initial breach was discovered in April this year, but that the intruders were likely present on the network from May 2014.

An inter-agency task force has been conducting a forensic investigation since the breaches were disclosed in June.

Archuleta faced hostile questions from lawmakers at several congressional hearings but defended her record, saying new systems she implement helped discover the breaches.

Some analysts have cited evidence pointing to China and have said the breach appears to be part of a wide-ranging intelligence operation which could gather sensitive data for recruitment, blackmail or extortion.

The White House said the incident underscores the need for new cyber-security legislation.