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.