The attorney for Viktor Bout called the prisoner exchange "fair" in a statement Thursday.

"As we have urged for some time, given the fifteen long years that Viktor Bout has been in custody since the United States government targeted him in 2006, his exchange for Brittney Griner, who has only been in custody for a few months, is fair," the attorney, Steve Zissou, said. "Like Brittney Griner, Viktor Bout will soon be reunited with his family."

Zissou thanked the Russian foreign ministry and Russian President Vladimir Putin for Bout's release from prison.

"Hopefully, this is just the first of many reasonable agreements between the U.S. and Russia that will lead to better relations and a safer world," Zissou said.

The exchange has drawn concern from some U.S. lawmakers, as well as criticism from the Drug Enforcement Administration agent who oversaw the agency's investigation of Bout.

Video released by Russian state media on Thursday shows Brittney Griner boarding a plane in Russia and saying she’s happy to be heading home.

The video was taken before Griner was released into U.S. custody.

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Brittney Griner boards plane in Russia before heading back to US

The video from Russian state media shows the WNBA star before she was released into U.S. custody.

Lindsay Colas, Brittney Griner's agent, said Griner "has carried herself with courage, grace and grit” and is expressing thanks to everyone who helped bring her home, including U.S. officials, the WNBA and the NBA.

Colas especially thanked President Joe Biden, who she said “kept his word."

“Also, to so many people around the world who raised their voices and stood with us - especially Black women, the LGBTQ+ community and civil rights leaders - thank you,” Colas said in a statement.

Colas said Griner will continue to advocate for other Americans held abroad and called people out by name, including Paul Whelan, who remains imprisoned in Russia, and Americans who are in Iran, Venezuela, China, Syria, Mali and Rwanda.

“Our commitment to President Biden and to the families of Americans who are being held hostage and wrongfully detained – especially Elizabeth and David Whelan, on behalf of their brother Paul Whelan, who remains in Russia and whose continued detention weighs heavily on our hearts – is to continue our work in the movement to bring them home,” Colas said.

The prisoner swap that resulted in Brittney Griner’s release has been in the works since at least Nov. 29, when the U.S. government requested the removal of Viktor Bout from USP Marion in Marion, Illinois, according to a court document unsealed Thursday.

The document, signed by George Turner, assistant United States attorney from the Southern District of New York, requested the Bureau of Prisons allow U.S. Marshals to take custody of Bout sometime between Dec. 2 and Dec. 16.

The move was “based on the significant foreign policy interests of the United States,” the document said, but did not mention Griner or a prisoner exchange.

The government expended significant resources and time to bring Bout to justice, but a source familiar with the prosecution of Bout said they recognized the "competing interests" that resulted in his release from U.S. custody.

Whether Bout represents a future threat, this source said that while Bout is not old he has been "out of the game," which would impede his ability to go back to his former operations. However, the source called Bout a "shrewd operator" with "a lot of friends" in Russian military and intelligence circles.

-ABC News' Aaron Katersky