WA, D.C. – A man was found dead of apparent carbon monoxide poisoning in a Northwest home, D.C. fire officials said.

D.C. Fire and Emergency Medical Services spokesman Alan Etter said firefighters were called to the 1000 block of Spring Road just after 9 a.m. Monday. Etter said someone, believed to be a family friend, entered the home, found the occupants unconscious, opened doors and windows to ventilate the house and called authorities.

Etter said EMS officials arrived and found the front door open. He said one man was found dead. Two other men and a woman were found unconscious in the home. They were transported to area hospitals in critical condition, Etter said. A male neighbor and a female neighbor also were hospitalized after feeling ill.

Firefighters took carbon monoxide readings inside the house, but Etter said they were not at lethal levels.

Etter said the D.C. Medical Examiner’s office will have to determine the exact cause of the victim’s death and the case will be investigated by the Metropolitan Police Department.

Police have not yet identified the man who died in the house.

Officials say that although elevated levels of carbon monoxide have been found in the house, they are still working to determine whether that substance was the actual cause of making the inhabitants sick. Authorities were also trying to find the source of the leak.

Hours after the incident the lead detective in the case was also feeling sick and was taken to the hospital for evaluation.

Monday afternoon, city inspectors deemed the house to be unsafe after finding several code violations.

The owner of the house had been working to correct the problems.