Russia’s Federal Security Service (FSB) announced the opening of a criminal case against a US citizen on suspicion of espionage on Thursday. The citizen was accused of collecting biological intelligence.

The FSB did not announce whether the suspect had been arrested, or the suspect’s name.

Russian authorities arrested and filed criminal charges against several Americans in recent years, and some were later released after in exchange for Russian citizens imprisoned in the United States.

Moscow and Washington have traded accusations of taking hostages for political purposes.