The brother of a US citizen arrested on Friday for spying in Russia has told BBC News he is innocent and was in Moscow to attend a wedding.
The family of ex-Marine Paul Whelan, 48, learnt of his arrest on Monday from news reports after wondering why they had not heard from him for days.
His twin brother David Whelan says he knew Russia well and cannot believe he would have broken any laws.
Russia's FSB state security agency says he was "caught spying" in Moscow.
The Michigan man was charged with the crime of espionage, for which he could be sentenced to at least 10 years in prison.
The US state department has requested consular access after being notified by the Russian authorities of the detention.
Spy scandals have erupted between Russia and America at regular intervals since the Cold War, while Russia's actions in Ukraine since 2014, and allegations of Russian meddling in the 2016 US presidential election, have seen relations plummet.
What was Paul Whelan doing in Russia?
The FSB says he was detained "during an act of espionage", a wording which implies that Mr Whelan was caught red-handed, the BBC's Sarah Rainsford reports.
In a statement tweeted by David Whelan, his family said it had been out of character for him not to be in contact even when travelling.
My brother was detained by the Russian government on Friday as an alleged spy. While the law library + info focus will remain, you may see an increase in off-message topics until we get him safely home. pic.twitter.com/2HIF1UmS1b— David Whelan (@davidpwhelan) January 1, 2019
Speaking to BBC News on Tuesday, David Whelan said his brother had arrived in Russia on 22 December and had been due to return on 6 January.
He had been attending the wedding of a fellow former Marine to a Russian citizen and had planned to visit Russia's second city, St Petersburg, in addition to Moscow.
Paul Whelan, his brother said, has been visiting Russia for business and pleasure since 2007, working in corporate security, with automotive industry components firm BorgWarner his most recent employer.
As well as serving in the US military, he has worked in law enforcement in the past.
Paul Whelan would stand out in a crowd, his brother suggested, as he is "about six foot [1.8 metres] and kind of hefty with a former soldier's build".
But asked if he could think of any reason why he had attracted the attention of Russian security services, David Whelan was adamant there was none.
"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.
"His innocence is undoubted and we trust that his rights will be respected," the family said in its statement.
How has the US responded to the arrest?
Paul Whelan's three siblings have contacted Congressional representatives, the US embassy and the state department, which David Whelan described as "very helpful".
"Russia's obligations under the Vienna Convention require them to provide consular access," a state department representative told BBC News.
"We have requested this access and expect Russian authorities to provide it. Due to privacy considerations, we have no additional information to provide at this time."
How extensive is spying between Russia and the US?
The two countries have been spying on each other for decades but very few US citizens have been arrested for espionage on Russian territory:
- In 2013, US diplomat Ryan Fogle was arrested and expelled after being accused of trying to recruit a Russian intelligence officer as a spy
The two countries have expelled each other's diplomats at intervals, notably last year over the nerve agent attack in the UK, which was blamed on Russia.
Last month, a Russian gun rights activist held in the US, Maria Butina, pleaded guilty to conspiracy. US prosecutors say she acted as a Russian state agent, infiltrating conservative political groups.
In 2010, 10 Russian agents were arrested in the US for deep-cover espionage and later swapped for four Russians convicted of spying for the West.