Russian authorities have formally charged an American with espionage, a move that could worsen already tense relations between the two countries.

Paul Whelan, a 48-year-old former Marine, was reportedly in Moscow to attend a wedding when he was detained on December 28. Russia’s domestic security services (FSB) initially said Whelan was apprehended while on a “spy mission,” and Russian media reported on Thursday that authorities have brought formal charges against him. Whelan is currently being held in one of Moscow’s most infamous prisons.

A State Department spokesperson confirmed to Vox Wednesday that US Ambassador Jon Huntsman had visited Whelan, and said that Huntsman had been in touch with Whelan’s family.

Secretary of State Mike Pompeo briefly addressed Whelan’s detention during a trip to Brazil on Wednesday. “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,” Pompeo told reporters.

Whelan’s family members, for their part, have dismissed allegations of espionage. In a statement, they said Whelan’s “innocence is undoubted.”

My brother was detained by the Russian government on Friday as an alleged spy. While the law library + info focus will remain, you may see an increase in off-message topics until we get him safely home. pic.twitter.com/2HIF1UmS1b — David Whelan (@davidpwhelan) January 1, 2019

Whelan’s detainment comes about two weeks after 30-year-old Russian national Maria Butina, who was arrested on charges of conspiracy and acting as an agent of a foreign government in July 2018, admitted to participating in a campaign backed by Russian officials to secretly influence US politics, including trying to sway the Republican Party to be more receptive to Russia. Some experts have speculated that Whelan’s arrest may be an attempt by Russian President Vladimir Putin to retaliate or set up a swap.

Whelan’s lawyer, Vladimir Zherebenkov, alluded to the possibility in an interview with the New York Times on Thursday. “This is a long process,” Zherebenkov said. “I myself hope that we can rescue and bring home one Russian soul.”

Whelan could face up to 20 years in prison if convicted for spying. (In contrast, Butina faces up to about six months in prison, and would likely be deported afterward.) There are still a lot of details about Whelan’s case that are unclear — but here’s what we know so far.

Who is Paul Whelan?

Paul Whelan’s family describes him as a former Marine and former law enforcement officer from Michigan, and a kind and loyal person who had an interest in Russia and Russian culture. The Kremlin claims the American was a spy — but so far has offered no real evidence to back up its explosive allegations.

Whelan served in the Marine Corps Reserve from 1994 to 2008 and, according to the Wall Street Journal, was deployed twice to Iraq, though he didn’t see combat. Whelan was discharged in 2008 for bad conduct, after being court-martialed on charges of larceny. The details of the case are not clear.

Whelan currently works as the director of global security for BorgWarner Inc., a Detroit-based auto parts supplier, according to his family and the company. “He was not on company business, it is our understanding he was on a personal trip,” Kathy Graham, a spokesperson for BorgWarner, told the New York Times. “We do not have any facilities in Russia.”

Whelan’s family said he flew to Moscow on December 22 with plans to attend the wedding of a fellow former Marine. David Whelan, Paul’s twin brother, told CNN that when his brother didn’t show up for the Saturday wedding, the couple filed a missing persons report. It wasn’t until Monday that Whelan’s family confirmed he had been detained by Russian authorities.

This wasn’t Whelan’s first trip to Russia. A Marine Corps publication dated November 2006 (per Quartz) printed a photo of Whelan in front of St. Basil’s Cathedral in Moscow’s Red Square, with a caption that said he was spending “two weeks of rest and recuperation leave in Moscow.”

On his personal website, Whelan wrote after that trip that it had been a “dream” to visit Russia. “I was fortunate enough to meet nice people and had several pleasant excursions throughout the country,” he wrote, according to the Daily Beast.

The Washington Post interviewed several of Whelan’s acquaintances in Russia, who described him as “a friendly man who greatly appreciated Russia,” with a basic command of the language. Whelan also maintained an account on Vkontakte, a Russian social media site. Images believed to be from his page show at least one post praising Donald Trump his inauguration in January 2017.

It looks like Paul Whelan, arrested in Moscow, has an account Vkontakte, (network under the FSB control). He posts congratulations on Victory Day in Russian, glorifies Trump & Russian army. The video on his page is dedicated to the Russian Defender of Fatherland Day. Versions: /1 pic.twitter.com/f6NUADsCcQ — Kseniya Kirillova (@KirkseniyaSF) January 3, 2019

Whelan’s brother David also told the Wall Street Journal that Whelan, having traveled to Russia before, was aware of the risks. Whelan, he said, “would not have knowingly broken any law, let alone one involving espionage.”

Was Whelan’s detainment revenge for Maria Butina?

US Ambassador Huntsman met with Whelan on Wednesday, five days after his arrest. But the details of why Russia thinks Whelan is a spy are still frustratingly unclear.

The timing of Whelan’s detention is certainly notable. A little more than two weeks ago, Russian citizen Maria Butina admitted that she tried to influence US politics by infiltrating Republican political circles and conservative-leaning interest groups — most notably the National Rifle Association. Butina was arrested in July but reached a deal with federal prosecutors in December and pleaded guilty to one count of conspiring to act as a foreign agent.

After Butina’s arrest in July, the Russian foreign ministry started a campaign in support of the Russian citizen — complete with the hashtag #FreeMariaButina. Putin previously claimed that he hadn’t heard of Butina until her arrest in the US, and denied she had any ties to Russian spies.

At a speech and press conference on December 20, though, the Russian president went further, claiming that the charges against Butina were concocted and that she pleaded guilty only under threat of a lengthy prison sentence. “I don’t understand what she could have pleaded guilty to because she was not there to fulfill any government tasks,” Putin said.

“We will see how it plays out,” he added. “We do care, and we will keep an eye on this case and provide our support accordingly.”

Putin’s apparent ire over Butina’s arrest has led some to speculate that Whelan’s detention is a form of retaliation — not an unusual move for the Kremlin. “Russia has a history of tit-for-tat diplomatic expulsions, and so forth, when there’s any kind of a spy case,” Kimberly Marten, a Russia expert and political science professor at Barnard College, Columbia University told me.

Marten added that Putin’s remarks about Butina in his December address also indicate that Whelan’s arrest could be a form of payback — or an attempt at a prisoner exchange, Butina for Whelan. But, she cautioned, “we don’t really know enough yet to say for sure.”

Former US Ambassador to Russia Michael McFaul called the situation strange on Tuesday and demanded Russia provide an explanation. Bill Browder, a longtime critic of Putin whose lobbying of Congress led to the passage of the Magnitsky Rule of Law Accountability Act in 2012, called Whelan’s detention “a hostage situation.”

The arrest of American Paul Whelan in Moscow by the FSB as he was visiting Russia to attend a wedding looks increasingly like a hostage situation. All foreign travellers to Russia should beware that anything could happen. The US government should intervene decisively in this case https://t.co/0qK6Uwmz5s — Bill Browder (@Billbrowder) January 1, 2019

Whelan’s lawyer didn’t rule out the possibility that Whelan could be sent back to the US in an exchange for Butina, telling the Times that he was open to a swap. He cautioned that, if this were the case, it could be a long process.

Experts have also noted that it’s pretty rare for Russia to detain a private US citizen, and that Whelan’s background definitely doesn’t scream spy.

“The one thing I can say for certain,” John Sipher, a former member of the CIA’s Clandestine Service, told NPR, “is this is not how the US commits espionage overseas. We would never put a US citizen, without diplomatic immunity, in harm’s way this way, especially looking after low-level things like this.”

At a press conference in Brazil on Wednesday, Secretary of State Pompeo said the US would demand Whelan’s release if his detention was inappropriate, but the Trump administration has otherwise been pretty quiet about the arrest.

That’s a bit surprising, since President Donald Trump has personally taken up the causes of Americans who were being held abroad in the past. He had success in returning American hostages where other presidents have failed, including Pastor Andrew Brunson, who’d been imprisoned in Turkey since 2016, and multiple Americans detained in North Korea. (The White House had not responded to Vox’s request for comment.)

Whelan’s arrest also comes as relations between Russia and the US are under strain. Reasons include the continuing Russian aggression in Ukraine, the fact that the US is still grappling with Russia’s interference in the 2016 elections, and the latest evidence that the Kremlin continued to meddle in the 2018 midterm elections.

In his speech on December 20, Putin also warned of the risk of nuclear war because of the US’s decision to abandon Cold War-era weapons treaties. The US has given Moscow until February to comply with the Intermediate-Range Nuclear Forces (INF) Treaty, and some experts think a breach could potentially jump-start an arms race.

Many of the details of Whelan’s arrest are still a mystery — along with Putin’s endgame, and whether this will set the US and Russia up for a showdown.