President Biden's prisoner exchange with Russia secured the release of WNBA star Brittney Griner on Thursday, but left U.S. Marine veteran Paul Whelan in Russian custody.

Whelan has been in Russian custody since 2018 when he was sentenced to 16 years in prison on espionage charges. Griner had been in Russian custody since February after authorities discovered vape cartridges containing a small amount of cannabis oil in her luggage.

Biden's administration had initially sought the release of both Griner and Whelan in exchange for notorious arms dealer Viktor Bout, a Russian citizen serving a prison sentence in America. Known colloquially as the "Merchant of Death," he was convicted in 2011 of conspiracy to kill Americans and sentenced to 15 years in prison.

The Biden administration ultimately agreed to exchange Bout for Griner alone, leaving Whelan behind in Russia.

Nevertheless, Whelan's brother, David Whelan, congratulated Griner on her release in a Thursday statement, but lamented the Biden administration's failure to release his brother.

"There is no greater success than for a wrongful detainee to be freed and for them to go home. 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," he wrote.

"She will be reunited with her family. Brittney is free. And Paul is still a hostage. But how many more times do I need to write that?" he continued. "Despite the possibility that there might be an exchange without Paul, our family is still devastated. I can't even fathom how Paul will feel when he learns. Paul has worked so hard to survive nearly 4 years of this injustice."

David's statement went on to note that the Biden administration notified the Whelan family that Paul would be left behind in the exchange. The statement noted that it was the second time the Biden administration had left Paul behind in a prisoner exchange, with the last instance coming in April.

"How do you continue to survive, day after day, when you know that your government has failed twice to free you from a foreign prison?" David added.

He pushed the Biden administration to be more aggressive in its efforts to get Americans released, going so far as to say the U.S. should arrest more Russians for exchange.

"As the use of wrongful detentions and hostage diplomacy continues around the globe, it's clear the U.S. government needs to be more assertive. If bad actors like Russia are going to grab innocent Americans, the U.S. needs a swifter, more direct response and to be prepared in advance," David argued. "In Russia's case, this may mean taking more law-breaking, Kremlin-connected Russians into custody. It's not like there aren't plenty around the world."

Biden says Griner will be back in the U.S. on Friday. He also vowed that his administration will continue negotiating for Whelan's release, adding that Russia is holding him for "totally illegitimate" reasons.