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Hi Dad? This is your daughter, the spy... The FBI is on to us. Sorry, I've got to run

When the film of Anna Chapman's brief spying career is made, as it surely will be, the studio that acquires the rights to her story will find the crucial scene already written -- by a US intelligence source and the FBI.

Between them, these sources shed new light yesterday on why the FBI moved in to shut down a ring of deep-cover Russian agents when it did, days after a summit between Presidents Obama and Medvedev.

The moment that Ms Chapman realised her cover was blown and her fake life as a New York socialite was almost certainly over is detailed in an 18-page FBI affidavit released on the day that she was arrested.

The document describes the now celebrated "illegal" meeting with an undercover federal agent in a Manhattan coffee shop on June 26 and the agreement to pass on a forged passport to another supposed Russian spy. It relates how she was followed to a shop in Brooklyn where she was seen buying a Motorola handset "so as to avoid detection of her conversations".

The only question left unanswered was who she called before failing to appear at the rendezvous as arranged on June 27.

According to a security source who briefed The Washington Post, that person was her father, Vasili Kushchenko, a former KGB operative now working at the Russian foreign ministry. She then called a friend in New York. Both advised her not to pass on the forged passport.

It is clear that despite her efforts, both conversations on the new mobile telephone were intercepted by US Intelligence, presumably with the help of the National Security Agency US officials already knew that another member of the spy ring, identified by the FBI as Richard Murphy, "was in the process of going to France and then on his way to Russia", Eric Holder, the US Attorney-General, said at the weekend.

"The concern was that, if we let him go, we would not be able to get him back."