The House Intelligence Committee on Tuesday morning interviewed a key character in the sprawling saga surrounding the so-called Steele dossier, a compendium of unconfirmed opposition research into then-candidate Donald Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE.

The panel questioned David Kramer, a former State Department official who is now a senior director at the nonprofit McCain Institute, according to multiple sources familiar with the matter.

It was Sen. John McCain John Sidney McCainThe Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' Cindy McCain: Trump allegedly calling war dead 'losers' was 'pretty much' last straw before Biden endorsement MORE (R-Ariz.) who gave the FBI the dossier in December. McCain was tipped off to its existence by a former British ambassador to Russia, who at McCain’s request arranged for the author of the dossier, former British intelligence agent Christopher Steele, to meet with Kramer to show him “the pre-election memoranda on a confidential basis,” according to court filings.

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The dossier has been of keen interest to Republicans, who want to know if it was used as the basis for a surveillance warrant on any members of the Trump campaign. Democrats, meanwhile, want to know if the salacious allegations it contains, many of which link Trump to Russia, are true.

Some of the allegations in the dossier have been confirmed, while many others have been proven to be false. Others remain unconfirmed and likely unconfirmable.

The dossier, created by the opposition research firm Fusion GPS, was funded in part by the Democratic National Committee and Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE's presidential campaign. Trump has roundly rejected the memo as "fake."