“We are deeply concerned for his safety and well-being,” his family said in a statement. “His innocence is undoubted and we trust that his rights will be respected.”

Whelan’s twin brother, David, said Paul was in Moscow for a wedding of a fellow Marine, which took place at an upscale hotel in central Moscow on Dec. 28, the day he was detained.

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“It is inconceivable to me that he would have done anything to break the law in Russia,” David Whelan told The Washington Post.

By Russian law, foreigners found guilty of spying on Russia face between 10 and 20 years in jail.

A member of the U.S. government should have visited Paul Whelan in detention by now, according to Russia’s obligations under the Vienna Convention, which dictates that consular access must be provided within a 72-hour window from the time of arrest.

But the U.S. government shutdown may have hindered this process, said a person familiar with Whelan’s case, who spoke on the condition of anonymity because of the sensitivity of the case.

Although U.S. embassies are not closed during the shutdown, they are working with reduced staff. The person added that it could take months before the case is resolved.

Whelan’s arrest comes as tensions between Washington and Moscow continue to escalate over issues including election interference, the crises in Syria and Ukraine, and the poisoning of a former Russian spy in Britain.

Whelan works as the corporate security director for BorgWarner, an automotive parts supplier based in Auburn Hills, Mich., that has business contracts in Russia.

“BorgWarner has been in contact with the relevant U.S. Government authorities in order to help our employee and the U.S. government,” the company said in a statement.

Whelan is a regular visitor to Russia. According to Whelan’s brother and Russian acquaintances reached by The Post, he has been visiting Russia since 2007.

He enlisted in the Marines in 1994 and rose through the ranks to become a staff sergeant, serving two tours in Iraq, in 2004 and 2006, according to military records released to The Post. Whelan was discharged for bad conduct in 2008 after being convicted of several charges related to larceny, according to the records.

Several Russian acquaintances described Whelan as a friendly man who greatly appreciated Russia and had a basic command of the language.

An acquaintance in St. Petersburg said the pair had made plans to meet in that city around Jan. 1, but contact with the American abruptly stopped on the day of his arrest.

The timing of Whelan’s arrest — coming weeks after Russian gun rights activist Maria Butina pleaded guilty to Kremlin interference in the United States — has raised questions about a potential swap. The two countries do not have an extradition treaty.

The arrest and guilty plea by Butina, 30, has become a sharp thorn in the side of U.S.-Russian relations. The first Russian national to be convicted of seeking to influence U.S. policy in the 2016 election campaign, Moscow has gone to great lengths to paint Butina as a political prisoner.