The FSB, Russia's federal security service, said that Paul Whelan was arrested in Moscow "while undertaking a spy action".
The FSB said the American had been detained on Friday, but it gave no details of the nature of his alleged espionage activities. The agency in Monday's statement didn't elaborate beyond saying he was caught "during an espionage operation", which the New York Times reads as implying "he had been caught red-handed".
On December 28th, Russian authorities detained American Paul Whelan. Butina said in court that she'd been operating as a Kremlin agent as she befriended National Rifle Association leaders and influential US conservatives.
Earlier this month, Russian gun rights activist Maria Butina pleaded guilty to conspiring with a senior Russian official to infiltrate US conservatives. She confessed to befriending influential pro-gun groups such as the National Rifle Association, an effort Butina said was backed by the Kremlin. She is likely to face up to six months in prison and then be deported to Russian Federation.
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Russian officials denied Butina is one of their agents.
Alexander Mikhailov, a retired FSB officer, said the arrest reflected the effectiveness of Russian counterintelligence. "There were simply no grounds for it".
The agency said the American was detained "while carrying out an act of espionage".
Relations between Russian and the United States have been strained since Moscow annexed the Crimean peninsula from Ukraine in 2014.