A man who died after catching fire early Thursday in Northwest Washington may have accidentally burned to death when his oxygen tank malfunctioned, according to a D.C. Fire Department spokesman.

District police and fire officials are still investigating precisely what caused the fire, which occurred about 3:30 a.m. in a second-floor room in a house in the 400 block of Newton Place, three blocks east of Georgia and New Hampshire avenues.

Information on the victim was not immediately available. Authorities said he called 911 and said he was on fire, but was unconscious when firefighters arrived.

The man died at an area hospital after suffering a heart attack, said Lon Walls, communications director for the D.C. Fire Department.

A cause of death is pending the results of an autopsy, authorities said.

Officer Araz Alali, a D.C. police spokesman, detectives have not yet concluded whether the fire was an accident or set intentionally. But Walls said fire investigators are focusing on the oxygen tank, which was next to the victim when the fire started.

A woman who answered the door at the house said family members did not want to speak to a reporter. There was a wheelchair on the front porch and stickers next to the front door warning against smoking.

“Oxygen in use,” one sign says.

This post has been updated