In July, on Mr Putin's orders, Russian security services also won official permission to hunt terrorists abroad.

ECONOMIST: Russian poisonings

State-run media also posted a series of photos released by Russian security services that purportedly showed Mr. Fogle's detention.

WSJ: Russia to Expel U.S. Diplomat

An unnamed official told the AP news agency that US investigators were working with the Russian security services, the FSB.

BBC: Boston bombs: Memorial for slain officer Sean Collier