The family of an American man arrested on espionage charges in Russia last week denied Wednesday he was a spy and said he was only in the country to attend a wedding.

Paul Whelan, 48, the head of global security for a Michigan-based auto parts supplier and a former U.S. Marine, was detained by Russia’s domestic security services on Friday. His twin brother, David Whelan, said Paul was in Moscow to attend a wedding for a fellow Marine when he suddenly disappeared. David said his brother has been to Russia several times before.

Whelan's family tweeted a statement saying they "are deeply concerned for his safety and well-being. His innocence is undoubted and we trust his rights will be respected."

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David Whelan said he doubted his brother was a spy.

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

He said his brother previously worked for Kelly Services, which maintains offices in Russia.

The Russian spying charges carry a possible prison sentence of up to 20 years.

Whelan received a “bad conducted” discharge from the Marines, according to his official military personnel file obtained by Fox News. His dates of service were listed as stretching from May 10, 1994 to Dec. 2, 2009. David Whelan said his brother fought in Iraq on multiple tours of duty.

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The State Department said Monday it received formal notification from the Russian Foreign Ministry of the arrest and was pushing for consular access. David Whelan said the family was told by the U.S. Embassy in Moscow that American representatives have not been able to speak to Paul.

Secretary of State Mike Pompeo said during his visit to Brazil on Wednesday he hoped the U.S. would get consular access to see Whelan.

“With respect to Mr. Whelan who is being held, we are hopeful within the next hours we'll get consular access to see him and get a chance to learn more,” Pompeo said. “We've made clear to the Russians our expectation that we will learn more about the charges, come to understand what it is he's been accused of and if the detention is not appropriate we will demand his immediate return."

The arrest comes weeks after Russian citizen Maria Butina, 30, pleaded guilty to a charge of conspiring to act as a foreign agent. Butina, who was arrested in July, admitted to acting as a secret agent for the Kremlin in trying to infiltrate conservative U.S. political groups. Butina faces a maximum of five years in prison and could be deported after her release. She will most likely serve about six months due to a plea agreement.

Russian President Vladimir Putin has said the case is fabricated and Butina entered the guilty plea due to the threat of a long prison sentence.

Fox News’ Lucas Tomlinson and the Associated Press contributed to this report.