Almost exactly five years ago, Bob Mueller met Ukrainian President Viktor Yanukovych, then a client of Paul Manafort. At the time, the meeting drew little attention—just a Facebook post from the Ukrainian Embassy in the U.S. and a few short write-ups from wire services. But now, the moment has historical significance: It brought together two men who would later (indirectly) face off in a legal battle that’s shaking the White House and its allies. The FBI and Mueller’s spokesman both declined to comment for this story.

The meeting took place in Kyiv. The European Pressphoto Agency snapped a picture of them shaking hands, with Mueller wearing his trademark black suit and muted tie, and Yanukovych grinning.

Ukraine General Newswire reported that the two men discussed counterterror cooperation, which made sense: The pair met about two months after the Boston Marathon bombing.

“I would like to focus on the most important issue for us – the issue of combating terrorism,” Mueller said, according to a Ukrainian Embassy’s Facebook post. “I would like to say thank you for the assistance provided to us after the Boston Marathon.”

Yanukovych sounded pleased with the meeting, according to the Facebook post.

“Ukrainian-American cooperation efficiently develops in many spheres of mutual interest,” he said at the time. “Your visit is very interesting for Ukraine and relations between our law enforcement bodies have established good traditions of cooperation and communication in the course of 20 years. I am confident that there is a potential for further broadening of cooperation.”

Yanukovych also said Ukraine was close to signing an agreement with the European Union.

“There are some preparations left but I hope that we will fulfill everything and sign the Agreement,” he said at the meeting.

That agreement—which Ukraine never signed—was a key focus of Paul Manafort’s lobbying work for Yanukovych. Manafort retained two American lobbying firms, The Podesta Group and Mercury LLC, to try to persuade members of Congress and think tank staff that Ukraine was moving in the direction of the West, even though Yanukovych was close with Putin and had imprisoned his main political rival, Yulia Tymosheko.

The meeting with Mueller came at a time when Yanukovych was looking to burnish his reputation with the West. And John Herbst, the United States ambassador to Ukraine from September 2003 to May 2006, said Yanukovych was always open to cooperating with the United States—but with a catch.

“Yanukovych was happy to pursue cooperation with the U.S. except if it touched upon his core interests in Ukraine, which were related—among other things—to ensuring his friends got their share of the national pie, and things that might have ticked off the Kremlin,” Herbst said.

“He was out for the good of himself, his group, and also his country, starting with himself and his group,” Herbst continued. “And for that, having a good relationship with the U.S. was a counter to being overly dependent on the Kremlin.”

Yanukovych wasn’t the only Russia-friendly Ukrainian who Mueller met with on that trip. An archived page from the Ukrainian security service’s website says he met with Oleksandr Yakymenko as well. As head of the country’s security service in 2014, Yakymenko appears to have signed off on a violent crackdown that resulted in the death of Euromaidan protesters. After the protests, he fled to Russia, according to Interfax-Ukraine. On Feb. 26, 2014, the country’s prosecutor general announced Yakymenko was wanted for the murder of protesters.

“You can’t predict ahead of time who’s going to end up with blood on their hands,” said Evelyn Farkas, a former Defense Department official focused on the region, of Mueller’s meeting with Yakymenko.

The Kremlin cheered Yanukovych’s 2010 presidential election victory, and the United States considered those elections free and fair. Former U.S. diplomats told The Daily Beast that they essentially gave Yanukovych a clean slate. A few years into his presidency, however, American officials had largely soured on him. Publicly, Yanukovych had tried to balance friendliness toward the West and toward Russia, playing both sides against each other. But over the years, he built a reputation as an “authoritarian in democrat’s clothing.” As Freedom House detailed, he concentrated power among his family members and close personal allies.

“By 2013, people saw he was leading the country in a more authoritarian direction and so he wasn’t getting a lot of facetime with senior officials,” said Steve Pifer, America’s ambassador to Ukraine from 1998 to 2000. “My guess is he heard Mueller was coming. If Mueller had not asked for the meeting, Yanukovych probably would have proposed it.”

Mueller made other stops in Eastern Europe. On May 7, 2013—a month before visiting Ukraine—he visited Moscow “to discuss cooperation on the Boston bombing,” according to a 2014 Congressional Research Service report. Before Mueller’s Moscow trip, Obama and Putin agreed in a phone call to cooperate more closely on counterterrorism efforts, per the report.

Yanukovych has now become a central focus of Mueller’s investigation into possible coordination between Trump associates and the Kremlin. On April 2 of this year, Mueller’s team filed a memo in federal court showing that when Deputy Attorney General Rod Rosenstein authorized the special counsel probe, he specifically highlighted Yanukovych (PDF). The memo said Mueller was specifically authorized to investigate allegations that Manafort “[c]ommitted a crime or crimes arising out of payments he received from the Ukrainian government before and during the tenure of President Viktor Yanukovych.”

And on Thursday, The Guardian reported that Manafort directed American lobbyists to smear Yanukovych’s foes in the United States—without filing the lawfully required lobbying disclosure forms. Last year, Manafort was indicted in federal court on a number of counts, including charges related to the work he did for Yanukovych.

The Mueller-Yanukovych meeting didn’t surprise numerous experts on the region who spoke to The Daily Beast. And it has likely benefited Mueller in his investigation.

“Mueller will be as well positioned as any human being on earth to obtain full cooperation,” Jonathan Winer, former deputy assistant secretary of state for international law enforcement, told The Daily Beast. “They would know him and they would trust him.

“To the extent they’ve got the same people in place, he’s going to know how to push the levers, push the buttons to achieve maximum cooperation, which the Ukranians would be likely to give the U.S. under almost any circumstances,” Winer added.

Herbst also said the experience would likely benefit Mueller.

“He’s not quite a neophyte in this particular area,” Herbst said. Whether it yields anything concrete is a whole different question. But it’s certainly a plus.”