Investigators have pinpointed the causes of two deadly D.C. house fires that happened within nearly a week of each other. A fire Monday night in a duplex on 10th Street in Northeast D.C. started when an extension cord overheated. D.C. Fire and EMS spokesman Vito Maggiolo said the cord was used to plug in a portable heater that should have been plugged directly into a wall outlet. An elderly woman rescued from the building’s upper floor later died from her injuries. A family member told a D.C. fire official the woman turned 94 in November. Last Tuesday, a Feb. 11 blaze at a Southeast D.C. home on Darrington Street also claimed the life of an elderly woman. Firefighters said she became trapped in a bedroom, and clutter in the home hindered their rescue efforts. Maggiolo said that fire was caused by smoking in bed.