Updated

WNBA star Brittney Griner has been released from the Russian prison system in exchange for Viktor Bout, a Russian arms dealer known as the "Merchant of Death," Fox News confirmed Thursday with a senior administration official. 

Reports earlier this year suggested that Bout, whose weapons sales fueled deadly conflicts around the world, could be exchanged for Griner and former U.S. Marine Paul Whelan.

But Whelan does not appear to have been a part of the exchange despite the assertion by the State Department that he has been wrongfully detained.

The former Marine was sentenced to 16 years in prison in 2020 on alleged espionage charges.

Russian officials have long pushed for the release of Bout, who was serving a 25-year sentence in U.S. prison after being convicted in 2011 of conspiracy to kill Americans, conspiracy to deliver anti-aircraft missiles, and aiding a terrorist organization.

He was nabbed in 2008 in a sting operation at a luxury hotel in Bangkok, Thailand, where he met with Drug Enforcement Administration informants who were posing as officials with the Revolutionary Armed Forces of Colombia, which has been classified by US officials as a narco-terrorist group.

Viktor Bout

Alleged Russian arms dealer Viktor Bout walks past temporary cells ahead of a hearing at the Criminal Court in Bangkok. (CHRISTOPHE ARCHAMBAULT/AFP via Getty Images)

Prosecutors said that Bout was prepared to provide the group with $20 million worth of "a breathtaking arsenal of weapons — including hundreds of surface-to-air missiles, machine guns and sniper rifles — 10 million rounds of ammunition and five tons of plastic explosives."

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Bout, who was played by Nicolas Cage in the 2005 movie "Lord of War," has maintained his innocence, claiming that he is a legitimate businessman. 

The Russian state-owned news agency TASS originally reported in May that talks are underway to exchange Bout for Griner. 

Viktor Bout, aka the Merchant of Death

Alleged Russian arms dealer Viktor Bout is escorted by policemen as he arrives for a hearing at the Criminal Court in Bangkok. (CHRISTOPHE ARCHAMBAULT/AFP via Getty Images)

Russian officials see Bout as a "high-value asset" and are likely pushing hard for the exchange, according to former Defense Intelligence Agency officer Rebekah Koffler. 

"Moscow wants him back because he possesses critical insights that he can share with the GRU, his former agency. Having been in a U.S. prison and interrogated by U.S. officials, he knows what our intelligence requirements are and other information that is valuable for the Russians," Koffler, the author of "Putin's Playbook: Russia's Secret Plan to Defeat America," told Fox News Digital. 

"It would be a big mistake for the U.S. to give up Viktor Bout, as much as one feels compassion for Ms. Griner and Mr. Whelan."

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The Biden administration confirmed Wednesday that it has made a "substantial proposal" to help free Griner and Whelan. 

"The U.S. government continues to work aggressively, pursuing every avenue, to make that happen and as part of those efforts we made a substantial proposal to bring Brittney Griner and Paul Whelan home," a White House spokesperson told Fox News on Wednesday. 

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Griner was arrested at a Russian airport in mid-February after police said they found cannabis oil in her bags. She pleaded guilty this month to a drug smuggling charge and could face up to 10 years in prison, but she has said that her "intent" was not to violate Russian law.

Whelan was sentenced to 16 years in prison in 2020 on espionage charges, but he and his family have maintained his innocence, while the U.S. government has called the charges false. 

The Associated Press contributed to this report.