Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.) on Thursday called on Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill Second GOP senator to quarantine after exposure to coronavirus GOP-led panel to hear from former official who said Burisma was not a factor in US policy MORE to turn over information about an unnamed informant that the Wisconsin Republican claims told him about secret "off-site" meetings of FBI officials.

McCaskill sent a letter to Johnson, the chair of the Senate Homeland Security and Governmental Affairs Committee, demanding "all information and documents" related to his investigation into bias and possible conspiracy at the FBI after he made "serious and damaging allegations" about the agency on television.

"As a former prosecutor I understand fully the power of allegations in the public domain," McCaskill wrote. "You have now made serious and damaging allegations. I would assume that you would never make those kinds of allegations without serious and substantial hard evidence."

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Her letter comes as some Republicans raise concerns about potential bias against President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE within the ranks of the FBI, with some floating the notion in recent days that FBI and Justice Department officials may have formed a clandestine group that held meetings in which they plotted to undercut Trump.

That notion stems from text messages exchanged between Peter Strzok and Lisa Page, two FBI officials who have come under scrutiny in recent months for harboring what some Republicans have deemed an anti-Trump bias.

One of those text messages makes reference to a "secret society." While the message has been presented without context, Republicans have seized on it to question whether FBI agents actively worked to undermine Trump.

Johnson stoked speculation on Tuesday when he said on Fox News that he had heard from an informant that FBI and Justice Department officials had held "off-site" meetings, though he later acknowledged that he did not what those alleged gatherings were about.

But McCaskill said that Johnson had not shared any information from or about the informant with Democratic members of the Senate Homeland Security and Governmental Affairs Committee. McCaskill serves as the top Democrat on that panel.

"If the Committee has any evidence that the FBI is, as you have stated, biased and corrupt at the highest levels, I assume that evidence is strong in both quality and quantity, and extends far beyond a casual mention in a text message between two agents who were involved in personal crosstalk," McCaskill wrote.