(Bloomberg) -- Russia’s Federal Security Service detained a U.S. citizen in Moscow on suspicion of espionage on Dec. 28, according to a statement on website.

The investigation department of the FSB, as the domestic security service is widely known, started a criminal case and investigations are underway, Interfax said, citing the FSB. It didn’t provide any details on where the person was being held.

The U.S. Embassy in Moscow was alerted about the detention in line with the bilateral consular convention, RIA Novosti reported, citing the Russian Foreign Ministry’s press office. The embassy has not yet issued a statement.

The detention comes amid heightened tensions between Russia and the U.S., including over Maria Butina, a Russian gun enthusiast who pleaded guilty earlier this month in the U.S. to conspiring to act as an unregistered foreign agent. Butina said in court that she’d been operating as a Kremlin agent as she befriended National Rifle Association leaders and influential U.S. conservatives.

Russian President Vladimir Putin spoke out about the case on Dec. 20, saying he didn’t understand why Butina was detained and that he would support her.

Spy scandals have also rocked diplomatic relations recently in other parts of the world. Earlier this month, a third Canadian citizen was detained in China. The detentions came as Beijing fumed over the arrest of Huawei Technologies Co. Chief Financial Officer Meng Wanzhou, who was arrested in Canada and then extradited to the U.S.

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