The breach was apparently carried out by hackers with connections to China and targeted a database containing copies of the government's Standard Form 86, according to news reports citing unnamed government officials. The form, available online, is a 120-page questionnaire that's answered by people seeking a national security clearance.
Those filling out the form are asked to provide highly personal details about their lives that go far beyond their birth dates and social security or passport numbers. Among the questions asked are details of former residences, names and addresses of neighbors and detailed information about family members.
There are also many questions, confined to the last seven years, that ask about contact with foreign nationals and problems with drug or alcohol abuse, debts or bankruptcy, imprisonment or run-ins with law enforcement.
The hack is believed to be separate from one disclosed last week that targeted the same agency and is feared to have resulted in the theft of personnel records on millions of current and former government employees in all branches of government except the military and intelligence fields.
That hack was initially thought to have affected about 4 million people, but the Associated Press reported on Friday that the actual number could be as high as 14 million.
The U.S. government has yet to officially pin blame for the first hack on any actor, but last week Sen. Susan Collins, a Republican from Maine and member of the Senate Intelligence Committee, said she believed the hackers were based in China.
On Thursday, a union representing government employees said it believes the detailed personal information stolen was not encrypted on OPM servers.
The Office of Personnel Management has yet to provide any details on the hacks beyond an initial statement published last week.
Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is martyn_williams@idg.com