WASHINGTON — Monica Crowley, the Treasury Department’s assistant secretary for public affairs, committed “localized instances of plagiarism” in her 2000 Ph.D. dissertation, Columbia University found in an investigation that ultimately concluded she did not commit research misconduct.

As part of the university’s review of Ms. Crowley’s work, she was required to make extensive revisions to her dissertation, a 493-page study of how American policy toward China evolved under Presidents Harry S. Truman and Richard M. Nixon. The research misconduct investigation, which concluded this month, was started after plagiarism accusations were raised about her work in 2017 after her appointment by Donald J. Trump, then the president-elect, for a senior National Security Council job.

“The investigation identified localized instances of plagiarism, but concluded that the preponderance of the evidence did not support a finding that Dr. Crowley committed research misconduct,” the university said.

Ms. Crowley, a former Fox News contributor and prominent conservative pundit, has served as Treasury Secretary Steven Mnuchin’s top spokeswoman since July.