CNN first reported that Horowitz had been investigating an FBI official who allegedly altered a document used to obtain a FISA warrant in order to surveil Trump campaign adviser Carter Page during the 2016 election.
"More broadly, Mr. Horowitz's report, to be made public on December 9, portrays the overall effort to seek the wiretap order and its renewals as sloppy and unprofessional, according to the people familiar with it".
The allegations are reportedly being levied by the Justice Department's inspector general, Michael Horowitz, who is releasing a report on December 9 reviewing the FBI's actions during the Russian Federation investigation. The CNN and Washington Post reports also cited unidentified individuals.
The Justice Department and the Federal Bureau of Investigation did not immediately respond to Insider's requests for comment.
The FBI obtained a secret surveillance warrant in 2016 to monitor the communications of Page, who was never charged or accused of wrongdoing.
Republicans have suggested for months they expect Horowitz's report to be highly damning, and at the very least will include calls for significant reform of the FBI's investigative procedures.
"I look forward to reviewing the report and hearing Mr. Horowitz's testimony before the Senate Judiciary Committee, where he will deliver a detailed account of what he found regarding his investigation, along with recommendations as to how to make our judicial and investigative systems better", read a statement from Graham's office. "There has been a continued cover-up to this day".
FBI Director Chris Wray has told Congress that he did not consider the FBI surveillance to be "spying" and that he has no evidence the FBI illegally monitored Trump's campaign during the 2016 election. Other sources say Horowitz's report may also shed light as to why USA attorney John Durham made a decision to open a criminal investigation into possible FBI misconduct. "It really was a dark cloud hanging over the campaign until the election in November of 2016, through the transition and going on years now into the early years of President Trump's first administration".
"I think they wanted to do much more than that too", Page said. "I hate to say it".