Paul Whelan, Imprisoned in Russia Since 2018, Is 'Disappointed' in U.S. Efforts to Secure His Release

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Paul Whelan — an American citizen and former Marine who was detained by the Russian government on espionage charges in 2018 — said he is "disappointed" that the Biden administration was unable to strike a deal for his release, but were able to do so for Brittney Griner.

Griner was released from Russian prison in a one-for-one prisoner swap for international arms dealer Viktor Bout on Thursday. The Biden administration had originally offered a "substantial" deal to trade both Whelan, 52, and Griner, 32, for Bout earlier this year.

In an interview with CNN from a Russian penal colony, Whelan said that he was surprised he was not included in the trade and said he had been "led to believe that things were moving in the right direction" and that "something would happen fairly soon."

"I am greatly disappointed that more has not been done to secure my release, especially as the four-year anniversary of my arrest is coming up," Whelan told CNN. "I was arrested for a crime that never occurred. I don't understand why I'm still sitting here."

He added that despite this, he is happy for Griner to be able to return home.

RELATED: Paul Whelan's Family Is 'Devastated' He Remains in Russia, but 'Glad' Brittney Griner Was Released

President Joe Biden said on Thursday his administration has not "forgotten about Paul Whelan," and alluded that bargaining for Whelan's release has been trickier.

"This was not a choice of which American to bring home," Biden said in a press release. "Sadly, for totally illegitimate reasons, Russia is treating Paul's case differently than Brittney's. And while we have not yet succeeded in securing Paul's release, we are not giving up.  We will never give up."

Secretary of State Antony Blinken added in his own statement, "While we celebrate Brittney's release, Paul Whelan and his family continue to suffer needlessly. Despite our ceaseless efforts, the Russian Government has not yet been willing to bring a long overdue end to his wrongful detention."

Biden noted that his administration would continue to keep in touch with Whelan's family and  negotiate for his release.

RELATED: Brittney Griner's Wife Cherelle Says 'Today My Family Is Whole' After WNBA Star's Prison Release

"It's a great day for the families of the wrongfully detained and we feel wonderful for them, but we do worry about what's in Paul's future," his brother David Whelan, said on CNN This Morning. "I think it's become clear that the U.S. doesn't have any concessions that the Russian government wants for Paul. So I'm not really sure what the future holds."

Officials told CNN that the exchange agreement between Griner and Bout was recently negotiated with Moscow and was given final approval last Thursday by Biden.

Griner's wife Cherelle said in a press conference at the White House that she was relieved that Griner was coming home.

"So over the last nine months, you all have been so privy to one of the darkest moments of my life, and so today I am just standing here overwhelmed with emotions, but the most important emotion I have right now is just sincere gratitude for President Biden and his entire administration," she said. "Today my family is whole."

She also recognized the Whelan family's continued concern, saying that  she will continue advocating for "getting every American home, including Paul, whose family is in our hearts today."

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Also on Thursday, American citizen Sarah Krivanek — who recently completed her unexpectedly harsh sentence in a Russian penal colony — boarded a plane to fly home.

She told PEOPLE in a harrowing exclusive interview that her imprisonment subjected her to "Sodom and Gomorrah"-like conditions, explaining why it's especially difficult for an American to endure time in Russia's labor camps.