The documents leaked by former NSA contractor Edward Snowden reveal that members of the terrorist network al-Qaeda reportedly applied for jobs at the US intelligence agency.

According to the documents, the US government reinvestigates thousands of employees a year to reduce the threat that one of its own may be trying to compromise closely held secrets.

The CIA found that among a subset of job seekers, one out of five had 'significant terrorist or hostile intelligence connections', Sydney Morning Herald reports.

With such a close threat of terrorists becoming part of the agency, the NSA planned to launch at least 4000 probes of potentially suspicious or abnormal staff activity after scrutinising trillions of employee keystrokes at work.

The report said that the CIA officials believed the number of applicants with potential links to terrorist networks or hostile foreign governments was 'small'.

The groups cited most often were Hamas, Hezbollah, and al-Qaida and its affiliates.

The US intelligence agencies' emphasis on such a practice came in light of the disclosures by WikiLeaks in 2010 which received hundreds of thousands of military and diplomatic documents from Army private Bradley Manning.

NSA spokeswoman Vanee Vines said that the investigations are part of an effort to try to reduce risks and not to investigate known threats.