Lengthy negotiations between the U.S. and Russia reached an accord on Thursday when the two nations exchanged Brittney Griner for arms dealer Viktor Bout, bringing the WNBA star home via private air lift.

But conspicuously absent from the plane was a different American — Paul Whelan — who is still being held in a Russian penal colony on charges of espionage.

Although Mr. Whelan, the brother of Island artist Elizabeth Whelan, had originally been included in negotations between the two countries, he was left out of the deal Thursday.

In a statement, Paul’s brother, David Whelan, said he was disappointed with the news that Paul had been left behind, but supportive of Ms. Griner and her family, who celebrated the news of her release with jubilance on Thursday.

“As the family member of a Russian hostage, I can literally only imagine the joy she will have, being reunited with her loved ones, and in time for the holidays,” David Whelan wrote. “The Biden Administration made the right decision to bring Ms. Griner home, and to make the deal that was possible, rather than waiting for one that wasn’t going to happen.”

Ms. Griner was arrested at a Russian airport ten months ago and charged with drug smuggling after Russian authorities found small amounts of marijuana in her baggage.

Reached by email, Ms. Whelan referred comment to the statement from her brother.

Paul Whelan, a former Marine, has been in Russian custody since December 2018, when he was arrested in Moscow on suspicion of international espionage. His family has continued to advocate for his freedom since his arrest, with Elizabeth Whelan leaving her quiet home on Chappaquiddick to lobby leaders in Washington, D.C. She met with President Joe Biden in September at the Oval Office.

Paul Whelan has 12 more years to serve in his prison sentence.

According to David, Paul has been in contact with the family every day since Dec. 2. He said that Paul had been optimistic about the negotiations, even worrying about where he would live upon his return to the U.S. David said the Biden administration had notified the Whelan family ahead of time to prepare them for the news Thursday.

“At some level, our family has steeled ourselves for this likelihood,” he wrote. “And I think, as the use of wrongful detentions and hostage diplomacy continues around the globe, it’s clear the US government needs to be more assertive.”

President Biden acknowledged the disappointment in a statement on Thursday, but asserted that the government would continue to try to negotiate for Paul.

“Sadly, for totally illegitimate reasons, Russia is treating Paul’s case differently than Brittney’s,” President Biden said. “And while we have not yet succeeded in securing Paul’s release, we are not giving up. We will never give up.”