NEW YORK (CNN) -- Columbia University said Thursday it has sanctioned for plagiarism the African-American professor on whose office door a noose was found last fall.

Madonna Constantine says she's innocent of plagiarism, but Columbia University defends its sanctions.

Madonna Constantine, professor of psychology and education at the university's Teachers College, was not dismissed, the university said.

Thursday's announcement came after an 18-month investigation, conducted by the law firm of Hughes Hubbard and Reed, said Marcia Horowitz, a Teachers College spokeswoman.

In a report presented in December to Teachers College administrators, the firm concluded that in two dozen cases, Constantine's published works contained language similar to passages in papers written by others, including a former teacher at the school and two of Constantine's former students.

In a statement addressed to the Teachers College community, the 44-year-old professor said she is innocent and called the action "premature, vindictive, and mean-spirited."

"It is my opinion that this investigation, along with other incidents that have happened to me at Teachers College in recent months, point to a conspiracy and witch hunt by certain current and former members of the Teachers College community," she wrote.

Constantine also accused college administrators of not following due process, a charge the college denied.

"She was given a full and fair opportunity to respond to the allegations against her," Horowitz said. "She was provided with copies of the journal works where similarities were found and was given an opportunity to explain them. All appropriate legal processes were followed."

Constantine co-wrote the book, "Addressing Racism: Facilitating Cultural Competence in Mental Health and Educational Settings." E-mail to a friend

CNN's Nkechi Nneji contributed to this report.

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