The Kommersant daily said it was probable that the suspected agent Ryan Fogle – officially a diplomat – was looking for information on the Tsarnaev brothers, who are alleged to have carried out the Marathon bombings.

It linked the Mr Fogle’s capture, which threatens to further stress ties between Washington and Moscow, to a trip that a US delegation made to Dagestan in April in coordination with the Russian authorities to investigate the blasts.

Suspected bomber Tamerlan Tsarnaev travelled to the region of Dagestan in 2012 and US authorities are eager to see if he built up contacts there with the local Islamist underground.

“It is likely that during the trip in April the US side obtained the phone numbers of (Russian) Federal Security Service (FSB) agents,” said Kommersant, which is known for its contacts in the security service and foreign ministry.