Under Russian law, espionage can carry a prison sentence of between 10 and 20 years. The Russian government said that the American were detained "during a special action". "All foreign travellers to Russian Federation should beware that anything could happen".
Paul Whelan had been staying with a wedding party for a fellow former Marine at the Metropol hotel in Moscow, said his brother David Whelan, who learned of the arrest on Monday.
David Whelan said he has no idea why his brother was targeted by the Russian security services.
Whelan's family said in a statement that his "innocence is undoubted" and they are "deeply concerned for his safety and well-being".
"Russia's obligations under the Vienna Convention require them to provide consular access".
Although US embassies are not closed during the shutdown, they are working with reduced staff.
-Russian ties are severely strained, in part over Russian meddling in the 2016 presidential election. Paul Whelan had traveled to Russian Federation in the past for work and to visit friends he had met on social networks, his brother said.
Mr Whelan declined to comment on his brother's work status at the time of his arrest and whether he lived in Novi, Michigan, as address records indicate.
A spokesperson for BorgWarner stated the company has no facilities in Russian Federation.
Whelan is a regular visitor to Russian Federation.
Paul Whelan did multiple tours in Iraq with the Marine Corps, his brother said. Mr Whelan's family say he's an innocent man.
An acquaintance in St Petersburg said the pair had made plans to meet in that city around January 1, but contact with the American abruptly stopped on the day of his arrest.
Putin told U.S. President Donald Trump in a letter on Sunday that Moscow was ready for dialogue on a "wide-ranging agenda", the Kremlin said following a series of failed attempts to hold a new summit.
The two countries do not have an extradition treaty and Russia has been known to arrest foreigners with an eye toward trading them for Russians held overseas.
The arrest came with Moscow embroiled in a number of spy scandals with the West and after President Vladimir Putin accused Western nations of using espionage cases to try to undermine an increasingly powerful Russian Federation. Butina is the first Russian national to be convicted of seeking to influence U.S. policy in the 2016 election campaign. The country's Foreign Ministry has gone to great lengths to paint Butina as a political prisoner, including launching a wide-ranging social media campaign.