(Adds comment from professor's lawyer)

By Nate Raymond

BOSTON, Sept 7 (Reuters) - The U.S. Department of Justice on Wednesday threw its support behind a lawsuit by three graduate students accusing Harvard University of ignoring sexual harassment by a professor who they said threatened their academic careers if they reported him.

The department in a court filing urged a federal judge in Boston to reject Harvard's claim that it could only be held liable for retaliation by the Ivy League school itself, not by any of its faculty members.

Harvard had made that argument in seeking to dismiss a lawsuit filed in February that claimed John Comaroff, an anthropology professor, for years kissed and groped students and threatened to sabotage students' careers if they complained.

The Justice Department in a filing supporting Margaret Czerwienski, Lilia Kilburn and Amulya Mandava argued that Title IX of the Educational Amendments of 1972, protects students' ability to report sex discrimination without fear of reprisal.

"For that to happen, schools must protect students who participate in the Title IX process from retaliation and respond effectively to known retaliatory acts of their employees," the department said.

Harvard did not respond to requests for comment. Comaroff, who is not a defendant, has denied harassing any student, and his lawyers said a review by Harvard did not find him responsible for retaliating against the plaintiffs.

In the lawsuit, the three plaintiffs they were among the students who reported Comaroff to Harvard officials. Yet despite those warnings, Harvard watched as he retaliated by ensuring the students would have "trouble getting jobs," the lawsuit said.

The Justice Department's filing came after Comaroff returned to the classroom Tuesday to teach his first course since being put on administrative leave in January, prompting protests.

"We're glad to see the government affirm that Harvard cannot skirt responsibility for the retaliatory actions of its faculty," Russell Kornblith, the three students' lawyer, said in statement. (Reporting by Nate Raymond in Boston, Editing by Alexia Garamfalvi, Richard Chang and Richard Pullin)