"The Russian Federal Security Service investigation department opened a criminal case against a United States citizen under article 276 of the Russian Criminal Code (espionage)".
The domestic security agency the detained individual as Paul Whelan.
Russian authorities detained a US citizen and accused him of espionage, Russian domestic intelligence agency FSB said in a statement Monday. No other details were given.
If convicted of espionage, Whelan faces up to 20 years in jail.
The security agency referenced a section of Russia's criminal code dealing with espionage, but did not provide any information about the specifics of the charges involved. She admitted to being involved in an organized effort, backed by Russian officials, to try to lobby influential Americans in the National Rifle Association and the Republican Party.
Russian President Vladimir Putin spoke out about the case on December 20, saying he didn't understand why Butina was detained and that he would support her. The country's Foreign Ministry has gone to great lengths to paint Butina as a political prisoner, notably by launching a wide-ranging social media campaign.
The announcement comes a day after Russian President Vladimir Putin said Moscow remained open to dialogue with Washington in a New Year's greeting to U.S. President Donald Trump.