When the U.S. government put out its latest sanctions list on Dec. 19, the man named at the top did not seem especially important. Described in the document as a former Russian intelligence officer, he was accused of handling money and negotiations on behalf of a powerful Russian oligarch. The document did not mention that the man, Victor Boyarkin, had links to the 2016 campaign of President Donald Trump.

A months-long investigation by TIME, however, found that Boyarkin, a former arms dealer with a high forehead and a very low profile, was a key link between a senior member of the Trump campaign and a powerful ally of Russian President Vladimir Putin.

In his only interview with the media about those connections, Boyarkin told TIME this fall that he was in touch with Trump’s then-campaign chairman, Paul Manafort, in the heat of the presidential race on behalf of the Russian oligarch. “He owed us a lot of money,” Boyarkin says. “And he was offering ways to pay it back.”