Cambridge firefighters responded twice Sunday to structure fires in the city.

In both cases, the cause remains undetermined.

"Both fires are still under investigation," said Fire Chief Jeff Deeks.

The first call was received shortly before noon. Firefighters found smoke coming from the duplex on South 11th Street, and all occupants out of the two-story structure.

Deeks said there was heavy damage to the second floor. No injuries were reported.

Firefighters received aid from city police, United Ambulance, AEP and Columbia Gas utilities personnel. As well, workers from the Cambridge street and water departments were on hand to deal with ice accumulation on roadways.

Firefighters cleared the scene and returned to the firehouse about 3 p.m.

Approximately four hours later firefighters responded to a report of a structure fire on North 11th Street.

Arriving at the scene they found smoke showing on the first floor of the two-story wood frame building, and found flames at the rear of the structure.

All occupants were out of the house upon the arrival of firefighters, and there were no injuries reported.

Firefighters returned to the station about 10 p.m.

As with the previous fire, firefighters received aid from city police, United Ambulance, AEP, Columbia Gas utilities personnel and workers from the Cambridge street and water departments, according to Deeks.

ddavis@daily-jeff.com