American basketball star Brittney Griner, who the US claimed had been wrongfully detained in Russia over alleged drug offences, was released on Thursday and was heading back home via the UAE, President Joe Biden said.
Mr Biden thanked his administration and the UAE, which enabled Griner's release along with Saudi Arabia.
“Welcome home Brittney,” Mr Biden said at the White House. “She's relieved to finally be heading home.”
The UAE's Ministry of Foreign Affairs and International Co-operation confirmed the country's role in the prisoner swap in a joint statement released with the Saudi Ministry of Foreign Affairs.
State news agency Wam said Griner had arrived on Thursday in Abu Dhabi on a private plane from Moscow, as Bout arrived on a private plane from Washington.
“The success of the mediation efforts was a reflection of the mutual and solid friendship between their two countries and the United States of America and the Russian Federation,” the statement added.
A senior US administration official told reporters that the US “leaned on countries around the world at various times in recent weeks and months to convey to the Russians how serious we've been about resolving wrongful detention matters”.
The release comes a day after Wam reported that President Sheikh Mohamed and Russian President Vladimir Putin had spoken by phone to discuss “issues of regional and international interest”.
Griner's wife, Cherelle Griner, said her family is once again complete.
“Today my family is whole. But as you all are aware, there's so many other families who are not whole,” Ms Griner said.
The Biden administration had also tried to secure the release of Paul Whelan, an American detained in Russia since 2018, but Moscow refused.
“BG [Brittney Griner] is not here to say this but I will gladly speak on her behalf and say that BG and I will remain committed to the work of getting every American home, including Paul [Whelan], whose family is in our hearts today as we celebrate BG being at home.”
Mr Biden said he would “never give up” fighting for Mr Whelan's release.
The Whelan family expressed their support for Griner's release but said they were “devastated” Mr Whelan was not included.
Mr Whelan was able to speak to his elderly parents early on Thursday morning. The Marine Corps veteran reacted stoically to the news, his sister Elizabeth said.
“My parents said he was amazingly resilient and courageous,” Ms Whelan told The National.
“I think that he himself has the strength to get through it. What it is, the uncertainty, is how the Russians will treat him over the long term and that, I think, is what we're all concerned about.”
Mr Whelan's brother David said in a statement: “Paul has worked so hard to survive nearly four years of this injustice.
“His hopes had soared with the knowledge that the US government was taking concrete steps for once towards his release. He'd been worrying about where he'd live when he got back to the US.”
US Secretary of State Antony Blinken thanked the US hostage release team for its work in securing Griner's freedom and extended his “deep appreciation” to “our Emirati friends, who assisted in the transfer today”.
Mr Blinken added that despite the US government's best efforts to free both Mr Whelan and Griner, “this was not a choice of which American. The choice was one or none.”
The Secretary of State told CBS News that he had “recently” spoken to Mr Whelan's sister.
He called the family “remarkable”.
Jonathan Franks, who worked on the earlier release of Trevor Reed, a US military veteran, welcomed Griner's release — but feared what it might mean for Mr Whelan.
Mr Reed, a former marine, was released in a separate prisoner exchange in April.
“I don't think we can pass off or completely discount the fact that they did this out of political expedience,” Mr Franks told The National.
“They wanted Brittney Griner home before Christmas and they got her, so I think that they can go get Paul.”
Ms Whelan believes that it is time for the Biden administration to start thinking “out of the box” to find solutions to get her brother home.
She has spent much of the past four years shuttling back and forth from her art studio in Massachusetts to Washington, where she has tirelessly advocated her brother's release.
With Bout off the table, it becomes a lot less clear how that might come about.
“We can't just keep doing what we've been doing,” she said. “If we're going to break this logjam, we've got to come up with some more creative thinking.”
Griner had been sentenced to nine years in prison and was moved to a penal colony last month.
The Phoenix Mercury basketball star, who played in Russia during the off-season, pleaded guilty to drug smuggling in July after customs officials found vape cartridges containing cannabis oil in her luggage at a Moscow airport in February.
Mr Biden called the sentence unacceptable.
Nicknamed the Merchant of Death, Bout had been serving a 25-year sentence after being convicted of conspiring to kill Americans and aiding a terrorist organisation.