Following the release of basketball star Brittney Griner from Russian captivity, US President Joe Biden faces fresh questions about another imprisoned American: former US marine Paul Whelan.
Mr Whelan, 52, was given a 16-year jail sentence in 2020 after being arrested in Moscow on suspicion of spying in 2018.
The Michigan native was not part of the prisoner swap which saw Ms Griner exchanged for convicted Russian arms dealer Viktor Bout - an omission that his brother has called a "catastrophe for Paul".
At a news conference on Thursday, President Biden said that the US government will "never give up" on securing Mr Whelan's release.
Here's what we know about Mr Whelan and his case.
Who is ex-Marine Paul Whelan?
Mr Whelan is a citizen of four countries - the US, Canada, the UK and the Irish Republic.
From Novi, Michigan, he was born in Canada to British parents and moved to the US as a child.
Military records show he joined the US Marine Reserves in 1994, about six years after he had reportedly begun work as a police officer in Michigan.
He started as an IT project manager for a company called Kelly Services in the early 2000s, but deployed with the marines within a couple of years on the first of two tours to Iraq, in 2004 and then 2006.
It was while serving in the marines that he made his first trip to Russia, a visit he detailed on his website, which was shut down several years ago. Sharing pictures of the visit, he said he had a "quite enjoyable time" exploring the cities of Moscow and St Petersburg.
But in 2008 Mr Whelan, who had achieved the rank of staff sergeant, was discharged from the marines for bad conduct.
The Pentagon, which released his records, said the charges related to larceny. CNN reports he was accused of using someone else's social security number, and "writing bad cheques".
His family, it has emerged, were unaware of events.
Mr Whelan continued to work for Kelly Services, being promoted to senior manager of global security and investigations in 2010.
According to court documents, he was in charge of the company's campus security group, and his duties included managing and conducting investigations, as well as handling some purchasing.
He left the company in 2016, moving to become director of global security for US-based automotive components supplier BorgWarner.
Mr Whelan was "responsible for overseeing security" at facilities in Michigan and around the world, a company statement said. It does not have any facilities in Russia.
Why is Whelan in prison?
Since his first visit in 2006, Mr Whelan made a number of other trips to Russia and developed an in-depth knowledge of the country.
It was due to his knowledge of Russia that he had been asked to attend the wedding of a fellow former marine in Moscow in December 2018, his brother David told the BBC's Newshour after his arrest.
"His experience of having already been to the Kremlin and having already walked around Red Square and navigating the metro, his friend felt Paul could assist a bunch of Americans who hadn't otherwise ever been to Moscow," David Whelan said.
On 28 December, however, he was arrested by Russia's FSB state security agency, which claimed he had been "caught spying" in Moscow. At the time, there were uncorroborated reports he had been caught receiving a digital storage device containing a list of intelligence officials.
There was also speculation over other possible motives for the arrest - including a suggestion Mr Whelan was detained so he could be exchanged with Maria Butina, a Russian gun rights activist, who was jailed in the US in December 2018. She was deported back to Russia in October 2019.
David Whelan, meanwhile, has dismissed the allegations as nonsense.
"I can't imagine how someone with a law enforcement background who is also a former US Marine, and who is now working in corporate security and is also aware of the risks of travel, would have broken any law let alone the law related to espionage," he said.
Why wasn't Whelan included in the prisoner swap?
Speaking in the White House Oval Office as he announced Ms Griner's release, Mr Biden said that "sadly and for totally illegitimate reasons, Russia is treating Paul's case differently than Brittney's".
The US government has been tight-lipped regarding efforts to release Mr Whelan.
In a 9 November statement - which largely focused on Ms Griner - the White House said that the US government is willing to follow up on its exchange offer for Ms Griner with "alternative potential ways forward" to secure the release of Americans detained in Russian, including Mr Whelan.
Alexei Tarasov, Viktor Bout's attorney, told Russian television that Mr Whelan wasn't included in the swap because the exchange was on a "one for one" basis.
"Really, an exchange should be equal," he said. "If we exchange one person, we should exchange him for one person, not for two".
What has the Whelan family said?
In a statement following Ms Griner's release, David Whelan said that the Biden administration warned the family ahead of time that Paul would not be released as part of the exchange.
"That early warning meant that our family has ben able to mentally prepare for what is now a public disappointment for us," the statement added. "And a catastrophe for Paul".
While David Whelan said the family does not "begrudge Ms Griner her freedom", the family believes it is "clear that the US government needs to be more assertive".
"If bad actors like Russia are going to grab innocent Americans, the US needs a swifter, more direct response, and to be prepared in advance," he added. "I can't imagine he [Paul] retains any hope that a government will negotiate his freedom at this point. It's clear that the US government has no concessions that the Russian government will take for Paul Whelan. And so Paul will remain a prisoner until that changes."