Russia's FSB domestic security agency said Monday it has arrested a U.S. citizen caught "during an espionage operation" in Moscow on Friday, but did not provide further detail. Russia's Tass news agency identified the alleged spy as Paul Whelan, noting that espionage charges carry a sentence of 10 to 20 years in prison. There has been no U.S. response to the arrest yet. Russia's spy announcement comes at a recent low point in U.S.-Russia relations; the U.S. has sanctioned Russia over its annexation of Ukraine's Crimean Peninsula and its interference in the 2016 election, and both countries have expelled each other's suspected intelligence agents. Two weeks ago, Russian gun-rights activist Maria Butina pleaded guilty to conspiring with Republican operatives as an agent of Moscow and agreed to cooperated with U.S. investigators.