Sussmann was charged in September with supposedly lying to the FBI and has pleaded not guilty.
The 27-page indictment says Sussmann told then-FBI General Counsel James Baker that he was not working for a client when he requested a meeting. At this meeting, he submitted evidence of what he insisted was a secret communication channel between Donald Trump and a Russian bank.
Though the document goes on to claim in fact, he was being paid by no less than two clients - a U.S. technology executive and the Clinton presidential campaign.
It proceeds to set out communications between Sussman and other figures.
"On or about September 15, 2016, Campaign Lawyer-1 exchanged emails with the Clinton Campaign's campaign manager, communications director, and foreign policy adviser concerning the Russian Bank-1 allegations that Sussman had recently shared with Reporter1," it states.
According to Fox News, the foreign policy adviser is Sullivan, 44, which cited two well-placed sources.
He has been a critical figure in Democratic circles since 2008 when he was a foreign policy adviser to Hillary Clinton's presidential run, before entering Barack Obama's general election team.
The Durham investigation seems to have entered a crucial phase. A grand jury indictment was returned last week for Igor Danchenko, a Russian analyst blamed for being an essential source in the Steele dossier.
He is charged with five counts of making misleading statements to the FBI.
There is no suggestion that Sullivan is a target of Durham's investigation, just that he got information from Sussmann.
The White House referred requests for comment to the Department of Justice.
"I'm not going to comment on that from here, from the podium," announced White House Principal Deputy Press Secretary Karine Jean-Pierre.
Trump has long denied any illegal conspiracy with Russia in his 2016 campaign, declaring that the charges were trumped up by his political enemies. Democrats insist that Durham's probe is a political hatchet job.
Durham was appointed as special counsel by the Trump Administration Attorney General Bill Barr in October 2020. He was tasked with examining whether the FBI's Trump-Russia probe, code-named "Crossfire Hurricane," was opened and handled legally.
The first criminal charges in Durham's inquiry were against Kevin Clinesmith, an FBI attorney assigned to the Robert Mueller probe, who changed an email during the process of acquiring a wiretap warrant renewal on Page.
Clinesmith pleaded guilty and was sentenced to probation.