Harvard police evacuated University Hall yesterday afternoon after a secretary received a phone call warning that a bomb had been planted in the building.

"At this point, we have no idea of who made the call," which reached the office of Associate Dean of the Faculty Robert A. Rotner, said Jack W. Morse, deputy chief of the Harvard Police. "But it will be worth 20 years in the penitentiary if we find out who did it," Morse said.

Morse would not comment on the sex of the caller, or on the police department's efforts to identify the caller. The secretary who received the call which prompted the scare was unavailable for comment.

Police arrived at University Hall after receiving a call from a supervisor on the building's third floor, according to Harvard police officer Alfred J. Doherty. Morse said the call reached the police station just after 2:30 p.m.

Police declared the threat "negative" after evacuating the building for approximately 30 minutes while three Harvard police officers and two Harvard security guards inspected it. Doherty said that evacuation is "normal procedure" for bomb scares.

Police then posted signs on the doors leading to the building, cautioning people to enter at their own risk. The signs were removed sometime in the evening.

Most workers disregarded the signs and re-entered their offices. "Some people may have left, but most of us are still here. We are using our own judgement," said Amalia Sgourakes, who works in the Office for Academic Planning.

According to Morse, yesterday's scare differs from many bomb threats at Harvard, which usually take place in response to demonstrations and controversies on campus. "This threat doesn't seem connected to any specific cause, but reading period tends to bring a few extra [threats]," he said.