An electrical fire and explosion in a manhole near the Peabody Terrace apartments yesterday emitted dangerous gases that forced residents to evacuate their homes for most of the day.

Officials said yesterday the cause of the fire was not yet known.

Fire officials responded around 10:30 a.m. to reports of foul-smelling smoke off Memorial Drive. Finding dangerous levels of carbon monoxide in some areas, they began to evacuate the buildings about half an hour later.

By 12:30, gases from a fire in an underground transmitter exploded, sending a fireball more than 70 feet in the air, said Deputy Chief Gerald Reardon of the Cambridge Fire Department (CFD). No one was injured in the blast.

Though fire alarms went off throughout Peabody Terrace, many residents did not leave their rooms. Officials said they had to go door-to-door to evacuate residents, who are mostly graduate students and their families.

Cambridge Electric, which is responsible for the transformer, shut off all power to the buildings while CFD contained the fire.

Residents were not allowed to return to their homes until carbon monoxide tests showed safe levels in each of Peabody's 500 units, and Cambridge Electric could provide power for the building.

"You literally have to go room to room, and that was the delay," said University spokesperson Joe Wrinn.