MANCHESTER, N.H. – The fire that killed a firefighter and a teen-ager apparently was caused by an overloaded extension cord, the state fire marshal’s office said Sunday.

The cord, loaded with appliances, was on the second floor of the building.

Meanwhile, a third victim remains in critical condition at a Boston hospital.

Patrick Flannery, 17, formerly of Newport, Maine, and veteran firefighter David Anderson, 43, died in the two-alarm fire. Flannery’s brother, Mathew, 12, was listed in critical and unstable condition Sunday.

An autopsy Sunday showed the boy died from smoke inhalation. The cause of Anderson’s death was not released. Investigators have said he died shortly after trying to rescue the two boys from their burning home Saturday morning. It was the city’s first on-duty firefighter death in 32 years.

Patrick Flannery was found dead in the home, while Mathew was rushed to the hospital. A third sibling, the boys’ mother and six other residents of the three-family home escaped safely.

The boys’ mother, Bea Flannery, said Patrick ran back into the house to try to save Mathew. Fire officials could not confirm her comments.

Patrick Flannery has been described by teachers at Memorial High School as quiet and respectful. He had transferred to the school in September from Maine.