The discovery, reported by The Washington Post, has heightened fears that America’s intelligence community is being targeted by terrorist and other extremist organizations for infiltration, as well as by foreign governments’ own spies. All such US agencies are investigating thousands of their own employees to counter the threat.

The classified budget document which revealed the extent of the CIA’s efforts was itself passed to the newspaper by the former National Security Agency contractor Edward Snowden, now a fugitive in Russia under temporary asylum after leaking thousands of secret documents.

Although the file did not describe the nature of the jobseekers’ extremist or hostile ties, it cited Hamas, Hizbollah and al-Qaeda and its affiliates most often.

The fear of infiltration is such that the NSA planned last year to investigate at least 4,000 staff who obtained security clearances. It detected potentially suspicious activity among staff members after trawling through trillions of employee keystrokes at work.

Suspect behaviour is said to have included staffers accessing classified databases they do not usually use for their work or downloading several documents, two people familiar with the software used to monitor staff told the Post.

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