Russia has opened a criminal case against a US citizen on suspicion of espionage, the country’s Federal Security Service announced on Thursday.

The security service did not name the individual or say whether he had been detained.

«The American is suspected of collecting intelligence information on biological subjects directed against the security of the Russian Federation,» the security service said. It did not further expand the accusations against the individual.

The State Department did not immediately respond to an overnight request for comment.

The development comes weeks after WNBA basketball star Brittney Griner was released from Russian custody after the Biden administration negotiated her release in exchange for arms dealer Viktor Bout.

Griner was among several US citizens detained in Russia in recent years. She returned to the US in December, nearly 10 months after she was first arrested and then jailed on drug charges.

While the United States was able to negotiate his release, other Americans remain in custody in Russia, including former Marine Paul Whelan, who was arrested in 2018 on espionage charges and sentenced to 16 years in prison in 2020.

Russia has so far refused to release Whelan, and the seriousness of the espionage charges is seen as one reason.

Last week, Russia released a US citizen who had crossed into its Kaliningrad enclave weeks after Moscow launched its invasion of Ukraine.

Taylor Dudley, 35, a US Navy veteran, was backpacking in Europe when he crossed the border between Poland and Russia in April, said Jonathan Franks, an attorney representing families of Americans detained in the abroad, according to Reuters.

The circumstances of how and why Dudley crossed the Russian border were unclear, nor were the details of how he came to be released.