Green Party candidate Jill Stein's campaign is now being looked at by the Senate intelligence committee as part of its investigation into Russia's interference in the 2016 presidential election.

Intelligence committee Chairman Sen. Richard Burr, R-N.C., confirmed to reporters on Monday that Stein's campaign was now a subject of his committee's work.

"I think it's safe to say we have two other campaigns we are just starting on," Burr told reporters, when asked if his committee was close to wrapping up its work.

Asked what they were looking for, Burr said: "Collusion with the Russians."

Burr declined to elaborate on what he meant by "two other campaigns." But his committee has also been examining reports that the Democratic National Committee paid the firm Fusion GPS for opposition research that went into the now-infamous dossier compiled by former British intelligence officer Christopher Steele.

Burr's confirmation of the expanded probe indicates that his committee's work will likely continue well into next year, even as the House intelligence committee looks to be winding down its separate investigation.

News that the Stein campaign was a subject of the Senate investigation was first reported by Buzzfeed News. Former Stein spokesman Dennis Trainor Jr. told Buzzfeed he had been notified by the campaign that the committee had requested a document search.

While not a major figure of the investigation so far, Stein has occasionally been invoked as one of the people who dined with Russian President Vladimir Putin and Michael Flynn, the former Trump national security adviser, at a 2015 dinner hosted by RT in Moscow. Flynn pleaded guilty last month to lying to the FBI and is cooperating with special counsel Robert Mueller's investigation.

"Our campaign has observed the highest standards of transparency and integrity in our interactions with foreign nationals as well as Americans," the Stein campaign said in a statement. "Our communications with Russian individuals regarding an invitation to speak on international relations at the RT 10th anniversary media conference will conﬁrm what we stated publicly at that time and since: that we did not accept any payment or even reimbursement for the trip, and that we made the trip with the goal of reaching an international audience and Russian ofﬁcials with a message of Middle East peace, diplomacy, and cooperation against the urgent threat of climate change, consistent with long-standing Green principles and policies."