(WASHINGTON) — In celebrating WNBA star Brittney Griner’s release, President Joe Biden made a point on Thursday to say the prisoner swap was “not a choice” between Griner and Paul Whelan, the American former Marine who is still detained in Russia.

“We never forgot about Brittney. We’ve not forgotten about Paul Whelan who has been unjustly detained in Russia for years. This was not a choice of which American to bring home,” Biden said from the White House.

“Sadly, or 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,” he said.

Whelan has spent four years in detention since he was seized in 2018 by Russia’s domestic intelligence agency, the Federal Security Service, while visiting Moscow for a friend’s wedding. He was sentenced to 16 years in prison on espionage charges but the United States and his family say they were fabricated in order to take him as a political bargaining chip.

The Biden administration remains in close contact with the Whelan family, Biden said, adding they must have “such mixed emotions today.”

“We’ll keep negotiating in good faith for Paul’s release. I guarantee that. I say that to the family. I guarantee you. I urge Russia to do the same to ensure that Paul’s health and humane treatment are maintained until we are able to bring him home. I don’t want any American to sit wrongfully detained one extra day if we can bring that person home,” he said.

Whelan’s twin brother, David, released a statement just ahead of Biden’s remarks calling the swap for Griner “the right decision.”

“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. 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 said.

He said officials let them know in advance “that Paul would be left behind,” unlike in April, when news broke of former Marine Trevor Reed’s return.

“This is the event we wish for so much for our own family,” the Whelans said in April, and again on Thursday. “She will be reunited with her family. Brittney is free. And Paul is still a hostage.”

The Whelan family is unsure if Whelan himself is aware of Griner’s release, his brother said, but they expect to speak with him soon, as they approach the fourth anniversary of his detention.

Griner will be back in the U.S. within 24 hours, according to officials.

“She’s safe. She’s on a plane. She’s on her way home,” Biden said alongside Brittney’s wife, Cherelle, Vice President Kamala Harris and Secretary of State Antony Blinken.

Blinken, in a written statement, praised Griner’s release but lamented that 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. I wholeheartedly wish we could have brought Paul home today on the same plane with Brittney,” Blinken said.

Former New Mexico Gov. Bill Richardson, who for months tried to help negotiate a trade for Griner and Whelan and travelled repeatedly to Moscow for meetings, said his team is thrilled Griner is now free but called for the Biden administration to now make Whelan’s return a priority.

ABC News’ Cindy Smith and Justin Gomez contributed to this report.

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