THE FBI has been accused of covering up a key case file detailing evidence against corrupt government officials and their dealings with a network stealing nuclear secrets.

The assertion follows allegations made in The Sunday Times two weeks ago by Sibel Edmonds, an FBI whistleblower, who worked on the agency’s investigation of the network.

Edmonds, a 37-year-old former Turkish language translator, listened into hundreds of sensitive intercepted conversations while based at the agency’s Washington field office.

She says the FBI was investigating a Turkish and Israeli-run network that paid high-ranking American officials to steal nuclear weapons secrets. These were then sold on the international black market to countries such as Pakistan and Saudi Arabia.

One of the documents relating to the case was marked 203A-WF-210023. Last week, however, the FBI responded to a freedom of information request for a file of exactly the same number by claiming that it did not exist. But The Sunday Times has obtained a document signed by an FBI official showing the existence of the file.

Edmonds believes the crucial file is being deliberately covered up by the FBI because its contents are explosive. She accuses the agency of an “outright lie”.

“I can tell you that that file and the operations it refers to did exist from 1996 to February 2002. The file refers to the counterintelligence programme that the Department of Justice has declared to be a state secret to protect sensitive diplomatic relations,” she said.

The freedom of information request had not been initiated by Edmonds. It was made quite separately by an American human rights group called the Liberty Coalition, acting on a tip-off it received from an anonymous correspondent.

The letter says: “You may wish to request pertinent audio tapes and documents under FOIA from the Department of Justice, FBI-HQ and the FBI Washington field office.”

It then makes a series of allegations about the contents of the file – many of which corroborate the information that Edmonds later made public.

Edmonds had told this newspaper that members of the Turkish political and diplomatic community in the US had been actively acquiring nuclear secrets. They often acted as a conduit, she said, for Inter-Services Intelligence (ISI), Pakistan’s spy agency, because they attracted less suspicion.

She claimed corrupt government officials helped the network, and venues such as the American-Turkish Council (ATC) in Washington were used as drop-off points.

The anonymous letter names a high-level government official who was allegedly secretly recorded speaking to an official at the Turkish embassy between August and December 2001.

It claims the government official warned a Turkish member of the network that they should not deal with a company called Brewster Jennings because it was a CIA front company investigating the nuclear black market. The official’s warning came two years before Brewster Jennings was publicly outed when one of its staff, Valerie Plame, was revealed to be a CIA agent in a case that became a cause célèbre in the US.

The letter also makes reference to wiretaps of Turkish “targets” talking to ISI intelligence agents at the Pakistani embassy in Washington and recordings of “operatives” at the ATC.

Edmonds is the subject of a number of state secret gags preventing her from talking further about the investigation she witnessed.

“I cannot discuss the details considering the gag orders,” she said, “but I reported all these activities to the US Congress, the inspector general of the justice department and the 9/11 commission. I told them all about what was contained in this case file number, which the FBI is now denying exists.

“This gag was invoked not to protect sensitive diplomatic relations but criminal activities involving US officials who were endangering US national security.”

An FBI spokesman said he was not familiar with the case file but he added: "if the FBI says it doesn't exist, it doesn't exist."

Insight: Chris Gourlay, Jonathan Calvert and Joe Lauria

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