Washington (CNN) Last month, an attorney expressed his outrage with leaders of the House Intelligence Committee: He demanded to know why a committee official shared his client's secret testimony with another lawyer, a blatant violation of the panel's rules.

Days later, the committee instead sent a subpoena signed by Chairman Devin Nunes demanding that the witness -- an associate to Sen. John McCain who had met with ex-British agent Christopher Steele -- reappear before the committee on short notice. News of the subpoena was reported by a conservative media outlet just 10 minutes after the witness received it.

The episode, which was described to CNN by four sources from both parties with knowledge of the meeting and which has not been previously reported, underscores the aggressive tactics Nunes and several of his senior staffers have employed to undercut Steele's dossier of allegations tying Donald Trump and his associates to Russia.

While Nunes, R-California, has rallied Trump and many Republicans to his cause by going after the FBI and Steele dossier, Democrats say he's stifled similar efforts to obtain more information on Russia and Trump, sitting on numerous requests for subpoenas, phone records and Democratic demands to schedule witness interviews.

The committee, instead, has issued subpoenas to the McCain associate and the firm behind the Steele dossier, Fusion GPS. And led by Nunes, the panel fought a lawsuit to secure Fusion GPS' bank records, quietly sent two aides to try to track down Steele in London and threatened to hold senior officials at the FBI and the Justice Department in contempt of Congress for not turning over documents.

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