There is a continuing criminal investigation into who placed the noose on the door, but there have been no charges in the case. When she was first found guilty of plagiarism, Dr. Constantine linked the two events.
I also never heard if there were any security cameras monitoring her hallway when the noose was hung. That question was kicked around for a time.
My understanding is that several cameras were in the building but not in the professors hallway.
Here's a pretty good background on the story.
http://www.villagevoice.com/2008-07-09/news/Columbia-Knotty-Noose-part-two/
The last I heard there were security cameras that had pictures of the “culprit” and they knew who it was. Then it was dropped.