Whelan family, Biden officials mark anniversary of former Marine's detention in Russia

Marking the four-year anniversary of Paul Whelan's detention in Russia, the Biden administration vowed on Wednesday to continue efforts to bring the former Marine home.

Whelan is serving a 16-year sentence at a Russian penal colony for espionage charges that the United States and his family say are baseless.

"His detention remains unacceptable, and we continue to press for his immediate release at every opportunity," Secretary of State Antony Blinken said in a statement on Wednesday. "I am committed to bringing home Paul and all U.S. hostages and wrongful detainees held around the world. As the President said directly to the Whelan family, our efforts to secure Paul's release will not cease until he is back home with his family where he belongs."

The White House's national security adviser, Jake Sullivan, similarly said Wednesday that officials "will not stop, we will not relent, we will not cease until all Americans can celebrate Paul's return."

The administration faced some criticism for failing to secure Whelan's release after WNBA star Brittney Griner -- who had been detained in Russia since February on drug charges -- was freed earlier this month in a prisoner swap for the convicted Russian arms dealer Viktor Bout, who was serving a prison sentence in the U.S.

U.S. officials had said they tried to trade Bout for both Griner and Whelan but Russia was not negotiating in good faith when it came to Whelan, who went on to work in corporate security after the Marines and who was arrested in a Moscow hotel in 2018 while visiting the country for a wedding.

"Sadly, for totally illegitimate reasons, Russia is treating Paul's case differently than Brittney's," President Joe Biden said as he announced Griner's release in remarks delivered from the Roosevelt Room on Dec. 8. It was either bring Griner home or no one, Biden said then.

Griner returned to the U.S. and was reunited with her wife, Cherelle, on Dec. 9. Griner later asked her supporters to advocate for Whelan's release, saying their efforts made an impact on her case.

"Together, we can do hard things. I am living proof of that," Griner wrote in a handwritten note posted to her Instagram account.

At the time of Griner's release, the Whelan family expressed support for the basketball star but said they were worried about his future and angry at Russia for, in their view, using him as a bargaining chip.

On Wednesday, Whelan's brother, David Whelan, also spoke out about the four-year anniversary of his detention.

"How do you mark such an awful milestone when there is no resolution in sight?" he said. "It is both awful and mundane, just another day that Paul has to suffer in a Russian labor colony for being in the wrong place at the wrong time."

"Another day that our parents have to suffer without being able to see or be with their son. It is both a culmination of lost life -- four years of missed birthdays, Christmases, and other experiences -- and also not an end point. It's merely a marker of their suffering, not an indicator that the suffering will come to an end before another milestone, another year passes," he continued.

David Whelan expressed gratitude for the government's continued efforts to bring Paul Whelan home.

"Hopefully these efforts will be successful," he said. "Hopefully Paul will be able to stop by our parent's home and visit them, free at last, before another milestone passes. On milestones, sometimes all you have is hope."

ABC News' Cindy Smith contributed to this report.