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WASHINGTON — Secretary of State Mike Pompeo said on Wednesday that the U.S. hopes to gain access soon to a former Marine who was arrested in Russia on espionage charges and that "if the detention is not appropriate we will demand his immediate return."

Paul Whelan, who is head of global security for a Michigan-based auto parts supplier, was arrested on Friday. In announcing the arrest three days later, the Russian Federal Security Service said he was caught "during an espionage operation," but it gave no details.

Whelan, 48, was in Moscow to attend a wedding when he suddenly disappeared, his brother David Whelan said Tuesday.

Pompeo, speaking in Brazil, said the U.S. is "hopeful within the next hours we'll get consular access to see him and get a chance to learn more."

Secretary of State Mike Pompeo speaks during a press conference at the United Nations in New York on Dec. 12, 2018. Kevin Hagen / Getty Images file

The U.S. has "made clear to the Russians our expectation that we will learn more about the charges and come to understand what it is he's been accused of and if the detention is not appropriate we will demand his immediate return," Pompeo said.

Whelan's family said in a statement David Whelan posted on Twitter, "We are deeply concerned for his safety and well-being. His innocence is undoubted and we trust that his rights will be respected."

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

David Whelan said in an interview that his brother had been to Russia several times previously, so when a fellow former Marine was planning a wedding in Moscow with a Russian woman he was asked to go along to help out.

The morning of his arrest, he had taken a group of wedding guests on a tour of the Kremlin museums. The last time anyone heard from him was at about 5 p.m. and then he failed to show up that evening for the wedding, his brother said.

"It was extraordinarily out of character," he said.

The family feared he had been mugged or was in a car accident, David Whelan said, and it was when searching the internet on Monday that he learned of the arrest.

"I was looking for any stories about dead Americans in Moscow, so in a way it was better than finding out that he had died," he said.

The State Department said Monday it had 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 they have not been able to speak to Paul Whelan.

David Whelan said he has no idea why his brother was targeted by the Russian security services. Paul Whelan had traveled to Russia in the past for work and to visit friends he had met on social networks, his brother said.

Paul Whelan did multiple tours in Iraq with the Marine Corps, his brother said. He now lives in Novi, Michigan, and is director of global security for BorgWarner, where he has worked since early 2017.

"He is responsible for overseeing security at our facilities in Auburn Hills, Michigan and at other company locations around the world," company spokeswoman Kathy Graham said in a statement.

She said BorgWarner does not have any facilities in Russia.

Paul Whelan previously worked for Kelly Services, which does maintain offices in Russia, his brother said.