(CNN) Former Milwaukee County Sheriff David Clarke must revise his master's thesis or risk losing his degree in security studies from the Naval Postgraduate School, according to documents CNN's KFile obtained through a Freedom of Information Act request.

The school arrived at its decision after a lengthy investigation triggered by a May CNN KFile report. The story revealed that Clarke's 2013 thesis, entitled "Making U.S. security and privacy rights compatible," contained language lifted from numerous sources, including multiple ACLU reports, the 9/11 Commission Report, The Washington Post, former President George W. Bush's book "Decision Points" and others. In all the instances KFile found, Clarke credited sources with a footnote but did not indicate with quotation marks that he was using the language verbatim.

Prior to the May story, Clarke said that he would be joining President Donald Trump's administration as assistant secretary in the Department of Homeland Security. In June, DHS announced that Clarke was no longer under consideration for the position. In August, he resigned his position as sheriff and announced the following week that he would join the pro-Trump super PAC America First Action.

The Naval Postgraduate School's dean of students, Cdr. Paul Rasmussen, wrote in a July letter to Clarke obtained by KFile that he concurred with a finding of an "Honor Code Board" that the thesis was "in violation" of the school's honor code. The dean wrote that he further determined that Clarke's "violation was not a result of any intentional deception or misappropriation efforts." The letter informed Clarke that he had until October 23 -- 100 days -- to submit a revised thesis or NPS would "initiate degree revocation."

Clarke did not respond to a request for comment on the school's decision.

When KFile reached out to Clarke for comment on its initial report, Clarke took to Twitter, writing, "This @CNN hack @KFILE oppo research MO is to accuse plagiarism. I'm next. Did it to Rand Paul, Monica Crowley et al." In an subsequent radio interview, Clarke said, "They're saying certain words and phrases I should have put quotation marks around. OK, alright, fine. Maybe from a formatting standpoint the thesis isn't perfect, but the content is there."

Lt. Cdr. Clint Phillips, a spokesman for the Naval Postgraduate School, wrote that the school had "no statement at this time. For us, the process was handled in a matter consistent with any other graduate or student."

The Naval Postgraduate School removed Clarke's thesis from its website shortly after CNN's initial report. The page where the thesis used to be now reads, "This item was removed from view at the discretion of the Naval Postgraduate School."

A prominent Trump supporter throughout the 2016 presidential campaign, Clarke gained notoriety for his inflammatory comments, such as calling Black Lives Matter a hate group.