According to the Russian authorities, he is a US spy. According to his family, he is simply a man who wanted to attend a friend’s wedding.
So what do we know about Paul Whelan, the 48-year-old who could face years in prison after being arrested in Moscow on suspicion of spying last month?
Mr Whelan, of Novi, Michigan, was born in Canada to British parents.
Military records show he joined the Marine Reserves in 1994, about six years after he had reportedly began work as a police officer in Michigan, in the US.
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 has been shut down. 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.
In his role Mr Whelan is “responsible for overseeing security” at facilities in Michigan and around the world, the company said in a statement. It does not have any facilities in Russia.
‘Generous person and loyal friend’
Since his first visit in 2006, Mr Whelan has made a number of trips to Russia. He has also reportedly set up a social media account on the Russian equivalent of Facebook, VKontakte.
It was due to his knowledge of Russia that he had been asked to attend the wedding of a fellow former Marine in Moscow on 28 December, his brother David told the BBC’s Newshour.
“Paul is a very generous person and a very loyal friend and in that role, a friend of his had asked him to come to Russia and help with the wedding party,” he explained.
“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.”
Russia’s FSB state security agency says he was “caught spying” in Moscow, and he has now been charged with the crime of espionage.
The BBC’s diplomatic correspondent James Landale says there are uncorroborated reports he had been caught receiving a digital storage device containing a list of intelligence officials.
There has also been 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 last month, our correspondent added.
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.