Paul Whelan, a former U.S. Marine and corporate security executive, is still being detained in Russia following Biden's successful prisoner swap of WNBA player Brittney Griner.

Griner was released Thursday in a swap for Russian arms dealer Viktor Bout. The professional athlete was detained in Russia in February for possessing vape cartridges with cannabis oil in her luggage. Bout was serving 25 years after being convicted in the U.S. for conspiring to sell weapons to a Colombian terrorist organization that intended to kill Americans.

Whelan, a U.S., Irish, British, and Canadian citizen, expressed his disappointment in the administration's efforts in a phone interview with CNN.

"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. I was arrested for a crime that never occurred," Whelan said. "I don't understand why I'm still sitting here."

Whelan was detained at his Moscow hotel in 2018 after attending a wedding in the country, convicted of espionage charges and sentenced to 16 years in a remote penal colony by a Russian court. The U.S. has repeatedly denounced the charges and criticized the court process involving Whelan's detainment.

Russia has reportedly refused to release Whelan, categorizing his alleged crime differently than other prisoners convicted of similar charges. He has compared his case to Griner's and Trevor Reed's. Reed was a former Marine detained in Russia on espionage charges in 2019 who was returned in a prisoner swap earlier this year.

"They've always considered me to be at a higher level than other criminals of my sort and for whatever reason, I'm treated differently than another individual here from a Western country that's also on a charge of treatment is also much different than others held for espionage at other prisons," Whelan said.

Biden has vowed to continue negotiating with Russia to return Whelan home.

Biden delivers remarks on the release of Brittney Griner and vows to bring fellow detainee Paul Whelan home.

"We have not forgotten about Paul Whelan, who has been unjustly detained in Russia for years," Mr. Biden told press in a White House briefing on Thursday. "We'll keep negotiating in good faith for Paul's release. I guarantee that. I say that to the family."

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

Whelan's brother said their family is "devastated" by his continued detainment. "It's a great day for the families of the wrongfully detained and we feel wonderful for them," David Whelan told CNN on Thursday. "But we do worry about what's in Paul's future. 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."

Paul Whelan pleaded with President Biden to bring him home in a message. "I would say that if a message could go to President Biden, that this is a precarious situation that needs to be resolved quickly," he said.

"My bags are packed. I'm ready to go home. I just need an airplane to come and get me."