While the Central Intelligence Agency does its own clearance investigations, agencies such as the State Department, Defense Department and National Security Agency, which eavesdrops on the world, all use OPM's services to some degree.
Intelligence veterans said the breach may prove disastrous because China could use it to find relatives of U.S. officials abroad as well as evidence of love affairs or drug use which could be used to blackmail or influence U.S. officials.
An even worse scenario would be the mass unmasking of covert operatives in the field, they said.
"The potential loss here is truly staggering and, by the way, these records are a legitimate foreign intelligence target," said retired Gen. Michael Hayden, a former CIA and NSA director. "This isn't shame on China. This is shame on us."
The SF 86 form, which is 127-pages long, is extraordinarily comprehensive and intrusive.
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Among other things, applicants must list where they have lived; contacts with foreign citizens and travel abroad; the names and personal details of relatives; illegal drug use and mental health counseling except in limited circumstances.
A review of appeals of security denials published on the web shows the variety of information now in possession of the hackers, including financial troubles, infidelities, psychiatric diagnoses, substance abuse, health issues and arrests.
"It's kind of scary that somebody could know that much about us," said a former senior U.S. diplomat, pointing out the ability to use such data to impersonate an American official online, obtain passwords and plunder bank accounts.
Some agencies less vulnerable
A U.S. official familiar with security procedures, but who declined to be identified, said some agencies do not use OPM for clearances, meaning their employees' data was at first glance less likely to have been compromised.
However, the former senior diplomat said someone with access to a complete set of SF 86 forms and to the names of officials at U.S. embassies, which are usually public, could compare the two and make educated guesses about who might be a spy.
"Negative information is an indicator just as much as a positive information," said the former diplomat.