The Republican chairman of the House intelligence committee has subpoenaed the head of the Washington firm that commissioned the sensational anti-Trump campaign research dossier — adding fresh intensity to the behind-the-scenes scramble among lawmakers to grasp how the infamous document fits into the Russian election meddling investigation.

The subpoena issued for Fusion GPS co-founder Glenn Simpson comes as special counsel Robert Mueller’s separate investigation interviewed the former British spy, Christopher Steele, who worked with Mr. Simpson in compiling the dossier of negative intelligence on Mr. Trump.

The CIA and FBI saw the dossier’s allegations as so sensational and sensitive last year that they excluded any mention of it from the intelligence community’s highly publicized accusations back in January about Russian meddling in the 2016 election.

Meanwhile, Carter Page, a former Trump campaign foreign policy adviser who filed one of several slander lawsuits because of the dossier, reportedly has said he will not appear before the Senate intelligence committee’s Russia probe. While unconfirmed whether the committee formally requested his testimony, the energy investor with Russia ties did inform the panel he won’t show if they do ask him.

Originally commissioned by Mr. Trump’s Republican rivals and then by a Democratic client, the dossier contains allegations about President Trump’s Moscow contacts. Mr. Steele wrote it in 2016 with support from Mr. Simpson and his firm before it was published by the online news service BuzzFeed just before January’s inauguration.

The Wall Street Journal first reported that House intelligence committee chairman Rep. Devin Nunes pushed for a subpoena for Mr. Simpson’s testimony, the latest sign that panel infighting continues to plague the House inquiry after months of investigating.

Last spring Mr. Nunes stepped down from heading the committee’s Russia probe amid allegations he failed to keep other panel members abreast of his activities. On Tuesday Fusion GPS lawyer Joshua Levy lashed out at him, questioning the legitimacy of the subpoenas and blasting them as a “blatant attempt to undermine the reporting” of the dossier.

Democratic committee members also slammed Mr. Nunes. Rep. Eric Swalwell, California Democrat, warned that “we are at risk of having a flat Earth report come out of the House intelligence committee as to what happened with Russia.”

Mr. Nunes has remained active in the investigation despite stepping aside and has also weathered criticism he is too close to the Trump White House. The Californian has countered that Democratic committee members have distorted the investigation to bash Mr. Trump at every opportunity possible.

The divisions mean the House committee could end up issuing competing reports, both GOP and Democratic sources close to the committee have told The Washington Times.

It was revealed last week that Mr. Mueller’s team spoke with Mr. Steele, a former MI6 agent, this past summer in Europe. The congressional probes have yet to talk with Mr. Steele, who started a private intelligence firm after leaving the British government.

Mr. Steele’s relationship with the American intelligence community has been at the center of the congressional investigations — particularly who specifically paid for him to compile the dossier — along with the depth of his ties to the FBI.

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