Sen. Chuck Grassley, chairman of the Senate Judiciary Committee, has sent a letter to FBI Director James Comey demanding the story behind the FBI's reported plan to pay the author of a lurid and unsubstantiated dossier on then-candidate Donald Trump.

Last month the Washington Post reported that the FBI planned to pay Christopher Steele to investigate Trump "a few weeks before the election." Steele was at the time being paid by supporters of the Hillary Clinton presidential campaign.

"The idea that the FBI and associates of the Clinton campaign would pay Mr. Steele to investigate the Republican nominee for president in the run-up to the election raises further questions about the FBI's independence from politics, as well as the Obama administration's use of law enforcement and intelligence agencies for political ends," Grassley wrote.

In his letter, Grassley demanded that the FBI turn over all its records relating to Steele and the dossier, in addition to "all FBI policies, procedures, and guidelines applicable when the FBI seeks to fund an investigator associated with a political opposition research firm connected to a political candidate, or with any outside entity."