Hackers that breached US government databases stole the personal information of at least 21.5 million people, officials said on Thursday.
Those affected include government job applicants, federal contractors, and over a million of their partners, the Office of Personnel Management said.
The figure is more than five times higher than the number of people that were feared to have been affected.
The data breach, which came to light in April, was widely blamed on China.
Authorities in Beijing have publicly denied any involvement.
The breach prompted a series of hearings in Congress and widespread criticism of the state of US cyber defences.
Last month, officials said personnel records of 4.2 million current and former federal government employees had been stolen in an incident.
