Robert Shonov, identified as a former employee of the US Embassy in Russia, was arrested in the Russian city of Vladivostok and charged with conspiracy, according to the Russian state news agency Tass.

Vedant Patel, a spokesman for the State Department, told reporters at a briefing on Monday that he had seen the report but that “I have nothing further to offer at this time.”

Tass, citing an anonymous law enforcement official, said Shonov was charged with “collaboration on a confidential basis with a foreign state or international or foreign organization.” He has been taken to Lefortovo prison in Moscow, Tass reported, and no trial date has been set.

Being held in isolation is common at Lefortovo, a notorious high-security prison, whose inmates currently include Evan Gershkovich, the Wall Street Journal correspondent who was charged with espionage in March, charges his employer and US officials have strongly denied. It is also where Paul Whelana former US Marine serving a 16-year sentence on what the US has said is fabricated charges of espionage, held for 20 months until trial in 2020.

During the Soviet era, the KGB held Soviet dissidents in the prison, and it has more recently been used to isolate opponents of the Kremlin.