A fire that killed a man inside his house in Northwest Washington on Thursday may have been an accident involving an oxygen tank, according to a D.C. Fire Department spokesman.

Lon Walls, communications director for the D.C. Fire and Emergency Medical Services Department, said investigators have ruled out a malfunction.

The spokesman said on Friday that the tank appeared to be working properly, and officials are trying to determine the precise cause, including the possibility of human error.

D.C. police identified the victim as 65-year-old George Martin III.

The small fire broke out about 3:30 a.m. in the second floor of a house in the 400 block of Newton Place, three blocks from Georgia and New Hampshire avenues.

Walls said the man called 911 to report that he was on fire, and he later died at a hospital after having suffered a heart attack. The cause of death is pending an autopsy.

A woman who answered the door of the house on Thursday said relatives did not want to speak to a reporter.

The victim is the sixth person to die in a fire in the District this year. There were six fire fatalities in all of last year, down from 12 in 2010, and 19 in 2009.