Police began collecting 56 hours of surveillance camera images they hope will reveal the culprit who strung a noose on a black professor's office door at Columbia University.

Investigators had downloaded about half the videotaped images late Thursday, police spokesman Paul J. Browne said. He said they planned to review tape from up to seven digital security cameras in and around the building where the noose was discovered early Tuesday morning.

Police also were testing the 4-foot-long piece of twine for DNA evidence and interviewing students and faculty, but said they had no suspects in the case.

Meanwhile, police were called to the Ivy League campus again Thursday to probe another distasteful discovery — a caricature of a yarmulke-wearing man and a swastika found on a bathroom stall door. The NYPD's hate crime unit was investigating the black-ink drawing, but police said there was no reason to believe the two incidents were linked.

University President Lee Bollinger said the sketch was promptly removed. He said he was reluctant to call attention to the drawings because he did not want to "broadcast, in any way, the message they attempt to send or empower those behind them."

Columbia's Teachers College turned over the security camera images in the noose investigation Thursday under pressure from police.

Browne said police officials were "disappointed and surprised" at a delay caused when the school refused to turn over the tapes without a court order.

Teachers College President Susan Fuhrman said in a statement that the school was eager to cooperate with police but asked for a court order because of concerns about students' privacy.

"There was no desire to hinder the investigation — far from it," she said. "This was a despicable act ... and we are doing everything we can to find out who did it and prevent such occurrences in the future."

Students, faculty and administrators denounced the attack on professor Madonna Constantine, 44, a professor of education and psychology who has written extensively about race.

Police said Thursday they had interviewed Teachers College professor Suniya S. Luthar after learning she had feuded with Constantine. But they said Luther was not considered a suspect.

Court records show Constantine filed a defamation lawsuit in May against Luthar, also a professor of psychology and education. The one-page filing in Manhattan's state Supreme Court also accuses Luthar of libel and slander and asks for $100,000 in damages.

Nooses are racially charged symbols of lynchings in the Old South and have appeared in a number of recent incidents around the country.

On Thursday, postal workers noticed a noose dangling outside a ground zero post office that was damaged in the Sept. 11 terrorist attacks, police and a postal official said.

Officials said it was not clear whether the noose was directed at the postal service or any other tenants in the lower Manhattan building. The U.S. Postal Service's Church Street station shares the building with the city Housing Authority, some state agencies and various other tenants.

The noose was found dangling from a lamppost near the building and was visible from the street, officials said.

"At this point, there was no target that was evident or any motive," U.S. Postal Inspection Service spokesman Al Weissman said Friday morning. He said no postal workers had reported any threats or other problems.

In other incidents, three white students hung nooses from a large oak tree outside a high school last year in Jena, La., fueling racial tensions. More recently, nooses have been found at the U.S. Coast Guard Academy in New London, Conn., and the Hempstead Police Department's locker room on Long Island.