Hackers who infiltrated the Office of Personnel Management and stole the security-clearance information of more than 18 million federal employees reportedly breached FBI agents’ personnel files in the process, Jeff Stein of Newsweek reports.

Some FBI employees were notified in May that their files had been accessed by hackers during the massive yearlong breach of OPM's database, an FBI source reportedly told Newsweek.

More than 36,000 people work for the FBI, and while it remains unclear how many were affected, access to any FBI personnel files could have “mind-boggling” effects, the source said, “because there are counterintelligence implications, national security implications.

he hackers' alleged theft of military and intelligence officials' SF86 forms from OPM could be used to blackmail, exploit, or recruit US intelligence officers.

The 120-page SF86 form is an exhaustive examination of the life of anyone applying for government security clearance, including financial records (with gambling addictions and any outstanding debt), drug use, alcoholism, arrests, psychological and emotional health, foreign travel, foreign contacts, and an extensive list of all relatives.

As members of the intelligence community, FBI employees' forms were stolen, too.

As a result, spies who took OPM information will know "who the best targets for espionage are in the United States,” Michael Adams, a computer-security expert with more than two decades' experience in the US Special Operations Command, told The Daily Beast.

“This is the most successful cyber attack in the history of the United States,” he added.

Hackers who infiltrated OPM had access to the agency's security-clearance computer system for over a year, the The Washington Post reported, giving them ample time to steal as much information as possible from OPM's database of military and intelligence officials.

Joel Brenner, who from 2006 to 2009 served as the Intelligence Community's top counterintelligence official, described the hack to AP as "crown jewels material, a goldmine" for China, adding, "This is not the end of American human intelligence, but it's a significant blow."

"I'm really glad to be out of the game," a recently retired CIA senior operations officer told former NSA intelligence analyst John Schindler in a Daily Beast article.

"There's bad, there's worse — and there's this," The former CIA officer said, referring to the breach. "CIA officers are not supposed to be anywhere in OPM files, but I'm glad I'm not posted overseas right now, hoping that's true."

Adams, the security expert who spoke to the Daily Beast, noted that the OPM hack is especially bad for people who hold the highest levels of clearance because those require more information in the background checks.

“The higher up you go in your sensitivity levels, the more data that’s in your adjudication file,” he said.

NOW WATCH: The US Navy just tested a giant electromagnetic catapult

See Also:

The South China Sea may be about to get even more crowdedThose Chinese hackers got into FBI files tooChinese hackers weren't just going after security clearance data ...

SEE ALSO: The US agency plundered by Chinese hackers made one of the dumbest security moves possible