A day after Columbia University fired a professor for plagiarism, police said they still don’t know who placed a noose on her office door in an incident that rocked the Ivy League campus.

RELATED: Professor Who Made Noose Claim Suspended.

Since the noose was discovered October 9, police have tested the 4-foot-long rope for DNA, conducted extensive interviews with possible witnesses, and reviewed tens of hours of security camera footage — but still have no suspects. A Manhattan grand jury is investigating the discovery, which sparked outrage last fall among students, faculty, and administrators.

The professor, Madonna Constantine, was a tenured professor of education and psychology at Columbia’s Teachers College and had written extensively about race.

She was sanctioned in February for plagiarism after the university determined she had used the work of others without attribution in papers published in academic journals over the past five years. She remained on staff and was allowed to appeal the ruling that she violated university standards, with her lawyer saying at the time she was targeted because she is black.

The plagiarism investigation began in 2006, well before the noose, a symbol of lynchings in the Deep South, was discovered on her door. A day after it was found, Ms. Constantine denounced the apparent hate crime at a raucous rally on campus.

Police at the time ruled out any possibility that Ms. Constantine had hung the rope herself. A police spokesman declined to comment yesterday on whether the timing of the incident was related to the school’s investigation.

A few weeks later, a swastika was discovered on the door of a Jewish professor at Teachers College. Police are still investigating that incident.