What happened at the University of Maryland?

A professor at the University of Maryland sent me a letter that the President of the university sent to the faculty. This is a passage: "Last night students leaving a program offered for Palestinian Awareness Week were confronted with anonymous disparaging posters and images that were attributed to no organization or person, and as such were in strict violation of University policy. They also violated the spirit of tolerance and open dialogue. Anonymous attacks are not open, not dialogue, and not defensible by people of character. The origins of these images are being investigated. "