Cambridge Electric officials have determined that a short circuit in an underground cable near Peabody Terrace caused the fire and explosion that forced residents to evacuate for more than eight hours on Sunday.

The Cambridge Fire Department (CFD) detected dangerous carbon monoxide gas in some apartments and ordered the evacuation. Shortly after residents began to evacuate, the built-up gas caused an explosion under a manhole.

A 120/208-volt cable connecting three transformers that deliver power to the 495-unit apartment complex short-circuited and caught fire, said Cambridge Electric spokesperson Eric de Lacoste. The rubber insulation burned for several hours in a fire that produced that poisonous gas.

De Lacoste said Cambridge Electric has not yet determined the cause of the short circuit. The utility is also unsure why carbon monoxide gas from the fire leaked into the complex, which mostly houses Harvard graduate students and their families.

"It should not have happened," de Lacoste said.

When workers replace the burnt section of cable, Cambridge Electric will investigate further.

Power was shut down at about 12:30 p.m. on Sunday so CFD could extinguish the fire, and was restored about four hours later. But residents were not permitted to return to their homes until almost 9 p.m. because CFD had to test carbon monoxide levels in each apartment.

Prompted by concerns from some residents who said they did not hear fire alarms, Harvard and city officials also checked the alarm systems before residents were allowed back into the building, said University spokesperson Joe Wrinn.

Deputy Fire Chief Gerald Reardon said there was "no question" fire alarms systems were working.

Officials had activated the alarms shortly after 11:30 a.m., Wrinn said, and began going door-to-door to make sure residents had evacuated the building.

"We found five to six people who said they heard the alarm but ignored it, which is obviously a problem," Wrinn said.

Reardon said it took several hours to search every room.

"We went through great pains to make sure that people who wouldn't come to the door got out," he said.

Peabody Terrace, located at the end of Banks Street, houses 1,300 graduate students, undergraduates, faculty and staff, many of whom had already left for intersession on Sunday.

Many residents had expressed anxiety Sunday about take-home exams they needed to finish but had left in apartments, and about exams scheduled for Monday.

No one called the Faculty of Arts and Sciences exam office to ask about delaying their test times. Five law students, however, asked that their Monday exams be rescheduled, according to the Law School registrar's office. The exams were deferred until yesterday.

Many international mid-career students at the Kennedy School of Government (KSG) live with their families in Peabody Terrace.

KSG Associate Dean Joseph J. McCarthy said each degree program would work out individual arrangements with students who had term-end projects due earlier this week. Affected students could reschedule a Monday in-class microeconomics exam, but McCarthy said he had not heard of other problems.