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.
A person familiar with the case was not authorized to discuss the matter by name and spoke to AP only on condition of anonymity.
"It will be December 9th, you'll get the report", the South Carolina Republican told Fox News. The Washington Post subsequently reported that the conduct of the Federal Bureau of Investigation employee didn't alter Horowitz's finding that the surveillance application of Page had a proper legal and factual basis, an official told the Post, which said the lawyer was forced out.
The Justice Department declined to comment to CNN on the matter.
Horowitz has said he will release the final report on his findings December 9, and he will face lawmakers two days later in a public hearing before the Senate Judiciary Committee.
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Also, the suit claims Smollett's passport was confiscated after his arrest and is yet to be returned almost nine months after his arrest.
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. "They have been falsifying documents and providing false testimony in the... courts of law for years now", he told FOX Business' Maria Bartiromo during an exclusive interview. "We still don't have the truth, but hopefully, we'll get that soon".
Attorney General William Barr has said he believed "spying" did occur, but he also made clear at a Senate hearing earlier this year that he had no specific evidence to cite that any surveillance was illegal or improper.
"I think they wanted to do much more than that too", Page said.
Mr. Horowitz gave the document to John Durham, a U.S. Attorney who is leading the investigation into the origins of the FBI's Russian Federation probe. "The first butts she booted this morning were Republicans who insist on floating weird conspiracy theories about the 2016 election", he said, and she testified that U.S. Ambassador Gordon Sondland was engaged in a "domestic political errand" for Trump in Ukraine. "I hate to say it". He will also sharply criticize as careless one of the F.B.I. case agents in NY handling the matter, they said.