Acknowledging that his committee had “hit a wall” when it came to assessing the credibility of the so-called Steele dossier — a controversial document that made a slew of Russia-related allegations against President Trump and his associates— Senate Intel Committee Chair Richard Burr (R-NC) made a personal entreaty Wednesday urging the dossier’s author to cooperate with the committee’s investigation.

Burr’s comments came during a press conference on Capitol Hill to provide an update on the status of the committee’s investigation into Russian interference in the 2016 election. Investigation of the explosive dossier compiled by former British intelligence officer Christopher Steele has been one area where the committee’s probe has stalled.

“We have, on several occasions, made attempts to contact Mr. Steele, to meet with Mr. Steele, to include personally the vice-chairman [Sen. Mark Warner (D-VA)] and myself as two individuals making that connection,” Burr said. “Those offers have gone unaccepted. The committee cannot really decide the credibility of the dossier without understanding things like, who paid for it, who were your sources and subsources.”

The dossier, which was published by Buzzfeed in January, became of interest to federal investigators last year. Buzzfeed and the private research company that was commissioned to put it together have been sued by Russian tech exec Aleksej Gubarev, who is among those mentioned in the dossier, and as part of the lawsuit, he is requesting more information about how the dossier came to be.

Burr said his committee has been “incredibly enlightened” in its “ability to rebuild backwards the Steele dossier up to a certain date” but is not likely to find the kind of intelligence products to understand its claims before that point.

“My hope is that Mr. Steele will make a decision to meet with with Mark and I, or the committee or both, so we can hear his side of it versus for us to depict in our findings what his intent or what his actions were,” Burr told the reporters at the press conference. “I say that to you but also say it to Chris Steele.”