Brittney Griner has asked her supporters in a social media post to advocate for Paul Whelan, a former Marine serving a 16-year prison term in Russia for espionage charges the United States are baseless.

The U.S. had sought to secure the release of both Whelan and Griner – the WNBA star detained at a Moscow airport in February for having vape canisters containing cannabis oil – but Russia agreed to swap only Griner for convicted arms dealer Viktor Bout. Griner returned to the U.S. earlier this month and Bout was sent to Russia.

In a handwritten note posted to her Instagram account on Wednesday, Griner told supporters their letters to her and advocacy for her release had made a big impact, and she asked them to do the same for Whelan, who was arrested in Russia four years ago. She included an address where supporters can send letters to the imprisoned American.

”I hope you’ll join me in writing to Paul Whelan and continuing to advocate for other Americans to be rescued and returned to their families,” Griner wrote.