(CNN) Former Milwaukee County Sheriff David Clarke has retained his master's degree in security studies from the Naval Postgraduate School after a lengthy revision process to correct multiple instances of plagiarism in his thesis.

The revision process was initiated last year after CNN's KFile reported that sections of the controversial sheriff's 2013 paper, titled "Making U.S. security and privacy rights compatible," were lifted word for word from sources ranging from the ACLU to former President George W. Bush. 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.

At the time of the initial report, Clarke had announced that he would be joining President Donald Trump's administration as an assistant secretary in the Department of Homeland Security. Clarke did not end up joining the administration. After resigning his position as sheriff in August, Clarke joined the pro-Trump group America First Policies, where he still serves as an advisor.

CNN's KFile reported in September that Naval Postgraduate School had informed Clarke that he had to revise the thesis by October 23 or the school would "initiate degree revocation."

Ultimately, it took Clarke 10 months after CNN's initial May 2017 story and several extensions of the resubmission deadline for his thesis to be revised to the satisfaction of the school, documents obtained by CNN's KFile through the Freedom of Information Act show. Clarke's revised thesis was accepted in March 2018.

"Matters of questionable academic conduct are not the norm on campus, but this case was handled as any other similar case would be," said Dale Kuska, director of communications for the school, in a statement to CNN. "We followed our instructions, as written, during the adjudication and Honor Board procedures, through the resubmission and acceptance of the thesis."

Clarke did not return requests for comment.

Internal NPS documents and emails show that frustration began to mount among the parties involved as early as August 2017.

That month, one school official, whose name is unknown because all school staff members' names were redacted from the documents CNN's KFile received, emailed another to say that Clarke believed he was "unfairly targeted and was never taught properly" how to cite sources. The official further recounted how Clarke had told him in a recent conversation that he did not understand why some of his revisions were inadequate and that the former sheriff said his eyes were "glazing over" at the official's attempt to explain.

In response, the other official appeared to scoff at Clarke's struggles, saying that "he could knock out the necessary revisions in a week of evenings if he stops trying to argue the case and just makes the changes."

The official added, "(Clarke) has dined out quite a few times on the 'I was never taught this stuff' cant, and I'm about done with it. The implication is that twenty-odd of his classmates somehow received the wisdom through osmosis or implication."

In December, following a review of another Clarke draft, officials wrote that "many of our comments were simply ignored" and that "some of his fixes are extremely superficial."

In emails of his own, Clarke also expressed irritation with what he called "a never ending process."

"It is unconscionable to now be told I need to GO BACK to the December version and find old errors," he wrote on December 18 after being told he needed to make further changes. "What would be the reason that I could not rely on the comments and corrections [redacted] requested after the October submission? This seems to be a never ending process that I will never be allowed to effectively complete. Please provide an acceptable explanation for what is happening here?"

"I am truly at their mercy and doing the best I can but it is taking a lot of time," Clarke wrote on January 28, in an email requesting additional time.

Despite Clarke's complaints about unfair treatment, however, Naval Postgraduate School officials were cautious in shepherding him through their process, with one writing to another in December that he or she had edited the latter's comments on Clarke's revised essay to ensure that they were consistent "with what we'd send to any other student."

Another December message from an Naval Postgraduate School employee said, "We've had to do more work than normal, but that is often the case with students given extensions after an honor board — following instructions and paying attention to details are not their strong suits."

Other emails suggest that the school was sensitive to the public nature of the matter, with one official requesting guidance on how to respond to a CNN Freedom of Information Act request. The reply, on which Clarke was added, stated that the school was permitted to inform CNN that Clarke had received an extension, but that "we would prefer you didn't answer them at all since it should be subject to student privacy".

Other documents hint at the involvement of the school's top leadership, with a February 1 email saying that the school's president, Ronald A. Route, personally granted Clarke one of the extensions.

When the process finally neared its end, both sides expressed relief.

"I don't know about you," one official wrote to another in March, "but I will be very happy when we get this one of (sic) our plate."

"This is the best news I have heard all year," Clarke said on March 13 after being told that the latest review had not identified any "significant plagiarism issues."

On March 30, after the thesis was reapproved, Clarke received a congratulatory email.

"David," it read. "Great to see this ended well. Your persistence and diligence has paid off! I don't know if you've seen it yet; [redacted]/NPS has been named #1 grad school for [redacted] education (recent U.S. News report). You can be proud of your NPS degree!"