The arrest of a US diplomat accused of being a CIA spy was linked to the Boston bombing, sources revealed today. 

Russian security officials reported on Tuesday that they had briefly detained Ryan Fogle in Moscow for allegedly trying to recruit a Russian intelligence officer.

Today sources revealed the man Mr Fogle was trying to ‘recruit’ was an FSB agent who specialised in Islamic extremism in Russia and may even have travelled to the region where the bombing suspects came from.

It is thought that he was part of a team who went to Dagestan and provided intelligence to the United States about an extremist threat in 2011.

Fogle, a third secretary at the U.S. Embassy, who was carrying special technical equipment, disguises, written instructions and a large sum of money. Fogle was later handed over to U.S. Embassy officials.

This morning the Russian foreign ministry today issued a formal protest to American ambassador Michael McFaul who was summoned to explain the alleged espionage mission of one of his diplomats. 

As he left after the brief session with Deputy Foreign Minister Sergei Ryabkov, the envoy waved to reporters but refused to comment.

The Ministry hit out at 'provocative acts in the spirit of the Cold War' and has ordered the expulsion of Fogle, arrested wearing a blond wig under his baseball cap. 

'This does not contribute to the further process of building mutual trust between Russia and the United States and bringing our relations to a qualitatively new level,' warned Dmitry Peskov, spokesman for Russian president Vladimir Putin.

But it was becoming clearer today that the US was seeking to lure into treachery an FSB agent who had knowledge of Russian intelligence operations on suspected Boston terrorist Tamerlan Tsarnaev, who lived in America but had travelled to Dagastan where he was believed to have met Islamic extremists.

The FSB had earlier warned the FBI about his potential extremist links. 

In material released by the FSB, it is clear the Americans had phone numbers for one or more Russian intelligence agents involved in anti-terrorism work in the Caucasus. 

They obtained these during trip involving FBI agents to Dagestan in search of intelligence on Tamerlan's trip.

'After the first call he refused to meet, but this man called again and insisted on a meeting,' said a recording of a FSB officer addressing three US diplomats who came to collect the alleged CIA agent from FSB headquarters.

'At first we did not believe it was happening, because recently the FSB has been actively helping to investigate the Boston blasts, and was also providing some other information about threats to US national security'.

Today Kommersant newspaper said: 'It is likely that during the trip in April the US side obtained the phone numbers of Federal Security Service (FSB) agents.'

'Clearly, they then decided to use it to have personal contacts with anti-terror agents, given that the exchange of information in the form of question and answers between special services is not always quick and smooth,' it said.

Russia has not named the target of the US co-operation, and it is not known whether the agent has faced any problems or even arrest over the US interest in him.

Fogle apparently hinted at an initial payment of $100,000 followed up a salary of up to $1 million a year plus bonuses if the Russian intelligence official handed over secrets to the CIA.

Russian Foreign Minister Sergei Lavrov said he had opted not to bring up the case at talks with US Secretary of State John Kerry on Tuesday in Sweden.

'I decided that talking about it would be superfluous, since it is already made public and everyone already understands everything,' he said.

Fogle was the first American diplomat to be publicly accused of spying in Russia in about a decade.

While relations between the two countries have been strained, officials in both Washington and Moscow sought to play down the incident.

Fogle was caught in Vorontsovski Park, an area in south-east Moscow, the FSB said.

A letter in Russian which Fogle carried suggests – if genuine – that the CIA hoped to reel in a big fish. 

Addressed ‘Dear friend’, it states: ‘We are ready to offer you $100,000 [£65,000] and discuss your experience, expertise and co-operation, and the payment may go much higher if you are ready to answer certain questions.

‘For long-term co-operation we offer $1million [£650,000] per year.’

The recruit is instructed to use an internet cafe to ‘create a new Gmail mailbox which you will use only for staying in touch with us’.

The incident is the biggest spy scandal since the arrest of glamorous agent Anna Chapman and nine other Russians in the US in 2010.

The FSB stated: ‘Recently, the US intelligence community has made repeated attempts to recruit employees of Russia’s law-enforcement bodies and special agencies.’

Many details remained shrouded in mystery last night. It is not known whether the target was part of the sting operation or if they have been arrested.

Russia’s haste to make the news public could mean either that the attempt was so audacious that it shocked leaders, or that hardliners have seized on it to stop a move towards detente with the US.

Yesterday Patty Fogle, the diplomat’s mother, refused to comment at her home in St Louis, Missouri.