The brother of a U.S. citizen arrested in Russia on espionage charges said Tuesday that he's innocent and was in Moscow to attend a wedding.

Paul Whelan, a retired Marine, was arrested in Moscow on Friday. The Russian Federal Security Service, in announcing the arrest three days later, said Whelan was caught "during an espionage operation," but gave no details.

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

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

Paul Whelan in Moscow in 2006. United States Marine Corps.

David Whelan said on MSNBC that his brother was part of a wedding party, and the friend who was getting married had asked him to lead tours around the Kremlin because he had been to Russia before.

"He loves to travel so I wasn't at all surprised that he would be confident going to Russia. He has a law enforcement background, he's got his Marine background, he does corporate security and he's aware of the risks of traveling in certain parts of the world," Whelan said.

The company Whelan works for, BorgWarner, an automotive parts supplier, said in a statement that Whelan serves as the company's director of global security and oversees a facility in Auburn Hills, Michigan, as well as other locations "around the world."

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A company spokeswoman said Whelan has worked for the company since 2017 and is responsible for securing facilities and assets. She added that BorgWarner does not have a facility in Russia.

Before that, Whelan worked for Kelly Services, a staffing company, according to a spokeswoman. Kelly Services is based in Michigan, but works with companies all over the world and does have a presence in Russia.

A Marine Corps article from 2007 said Whelan used the Rest and Recuperation Leave Program "experiencing the post-soviet era of Moscow and St. Petersburg, Russia."

The program gave service members a 15-day break, and paid for them to travel anywhere in the world. At the time, Whelan was a staff sergeant serving with the 3rd Marine Aircraft Wing in Iraq.

The article noted that most deployed service members took the opportunity to go home for free, but Whelan is quoted in the article noting that "it gives those of us who are single an opportunity to travel throughout the world wherever we want to go and experience the diversity of culture."

Whelan, 48, had been in Russia for a day or two before Friday when he stopped contacting his family back in the U.S., and then they learned that he had missed the wedding. The family learned of Whelan's detention from online news reports on Monday, his brother said.

"It just never would have occurred to me that A) he would have any sort of trouble in a large metropolitan area or B) that his background would suggest that he would be willing to commit any crime, let alone an espionage crime," said David Whelan.

He added that the embassy in Moscow was helping his family figure out what happened, but because of a 72-hour window that has to pass before they gain access to Paul Whelan, "no one, as far as I know, has seen or spoken to him yet."

The State Department said Monday it had received formal notification from the Russian Foreign Ministry of the arrest and was pushing for consular access. The department did not identify Whelan at the time or provide any information about the case, citing "privacy considerations."

Whelan said Tuesday his family was "doing the best we can in light of very little information and a rather implausible situation."

He said they have faith that his brother will come home safe. "I think Paul has a superior set of skills to most of us," Whelan said. "And so hopefully he won't have to [deploy] too many those skills in order to survive and to come home."

The arrest comes as U.S.-Russian ties are severely strained, in part over Russian meddling in the 2016 presidential election as well as the conflict in eastern Ukraine and Moscow's annexation of the Crimean Peninsula in 2014.

The U.S. and other Western countries have imposed sanctions against Russian officials, companies and banks.

A Russian gun-rights activist, Maria Butina, is in U.S. custody after admitting she acted as a secret agent for the Kremlin in trying to infiltrate conservative U.S. political groups as Donald Trump was seeking the presidency. She pleaded guilty in December to a conspiracy charge as part of a deal with federal prosecutors.

Russian President Vladimir Putin has claimed the case is fabricated and that Butina entered the guilty plea because of the threat of a long prison sentence.