ANN ARBOR, Mich. (AP) _ A roof over an underground chamber at the University of Michigan campus collapsed Thursday, but no injuries were reported, authorities said.

Three electricians working in the chamber beneath the Dow Building, which houses the university's School of Engineering, fled about 1 p.m. after hearing rumbling noises prior to the collapse, said Ann Arbor Police Capt. Donald Carnahan.

The Dow Building was evacuated, but Carnahan said police did not know how many people were inside. Between 50 and 75 people were outside when officers arrived, he said.

The underground chamber was part of a tunnel system connecting campus buildings, Carnahan said. Police did not know what caused the collapse and did not have a damage estimate immediately available, he said.