Story highlights James Comey implied allegations contained in a dossier could be subject to a federal probe

President Donald Trump and his supporters have dismissed the document

Washington (CNN) Senate judiciary committee Chairman Chuck Grassley threatened to subpoena the political firm that compiled a dossier at the center of the federal Russia probe.

Lawyers for Fusion GPS, a political research firm run by a former Wall Street Journal reporter, wrote to Grassley in April that they could not provide backup documents behind the dossier -- written by former British intelligence agent Christopher Steele -- because of client confidentiality agreements.

But Grassley responded this week he may force the firm to produce the records.

"While any confidentiality agreements may prevent Fusion GPS from complying voluntarily, they do not supersede the committee's constitutional authority to compel the production of information," Grassley, an Iowa Republican, said in a letter written Wednesday and released publicly Friday.

President Donald Trump and his supporters have dismissed the document -- with some fairly stunning allegations about Trump and his campaign -- as being "dodgy" and faulty.

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