The Senate Intelligence Committee has been unable to assess the credibility of the infamous Trump dossier because its author, Christopher Steele, has refused to cooperate with the panel’s investigation into Russian interference in the presidential campaign.

“Unfortunately, the committee has hit a wall,” North Carolina Sen. Richard Burr, the chairman of the committee, said during a press conference on Wednesday.

“We have on several occasions made attempts to contact Mr. Steele, to meet with Mr. Steele,” said Burr, who added that attempts made by he and committee vice-chairman Mark Warner, a Virginia Democrat, to meet with the former British spy have been unsuccessful.

“The committee cannot really decide the credibility of the dossier without understanding things like who paid for it, who are your sources and sub-sources,” Burr added.

Steele, a former MI6 agent, was hired by the opposition research firm Fusion GPS last June to investigate Donald Trump’s personal and business activities in Russia.

The 35-page document he authored, which consists of 17 separate memos, has become a focal point in the federal investigation into possible collusion between the Trump campaign and Russian government. The FBI has reportedly relied on the document to form the basis of its counterintelligence probe. U.S. officials also reportedly cited Steele’s work in an application for a surveillance warrant against former Trump campaign adviser Carter Page.

Steele’s memos, which date from between June 20, 2016 and Dec. 13, make a series of salacious allegations, including that the Kremlin has blackmail material on Trump and that members of the Trump campaign cooperated directly with Russian operatives.

Fusion GPS hired Steele to investigate Trump on behalf of a political ally of Hillary Clinton’s. Both Fusion and Steele, who operates a private intelligence firm in London, have refused to identify the client.

Burr made a direct appeal to Steele to meet with the committee. He said that though committee investigators have been able to “rebuild backwards” parts of the dossier up to a certain date, “getting past that point has been somewhat impossible.”

He said that he does not believe that the U.S. intelligence committee will be able to help.

“My hope is that Mr. Steele will make a decision to meet with either Mark and I or the committee…so that we can hear his side of it, versus for us to depict in our findings what his intent or his actions were,” Burr added.

Burr told reporters Wednesday that the committee has conducted more than 100 interviews totaling more than 250 hours during its probe.

More than 4,000 pages of interview transcripts have been compiled, and more than 100,000 pages of documents have been reviewed, the lawmakers said.

Burr and Warner provided little new insight into whether it is believed that members of the Trump campaign colluded with the Russian government.

“The committee continues to look into all evidence to see if there was any hint of collusion,” Burr told reporters, adding that he would avoid discussing preliminary findings on the issue because “we don’t have any.”

Burr said that 25 additional witnesses have been booked this month.

SENATE INTEL CHIEF: NO END IN SIGHT FOR RUSSIA INVESTIGATION:

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