A carbon monoxide leak in a downtown Shanghai apartment killed a man and a woman and left two other roommates in a coma Sunday morning, Shanghai police confirmed.

Police received report of the accident at 10 a.m., from a man surnamed Xu.

Xu said his friend -- one of the four victims -- had failed to show up for an appointment and phone calls were unanswered. "I was suspicious, so I went to his rented apartment in Putuo District."

When the door bell went unanswered, Xu forced open the door and found all the four tenants -- three men and a woman -- lying unconscious. "I smelt a stinging odor and guessed it could be a carbon monoxide spill."

Police confirmed one man was already dead and the other three victims were sent to hospital. The woman failed to respond to emergency treatment.

Investigators said an electric water heater, fueled by liquefied natural gas, had leaked carbon monoxide. The toxic gas had accumulated quickly in the apartment, whose windows were all shut.

As of 3 p.m., the other two victims were still in critical condition.

Police said all the victims were aged between 22 and 25.

(Xinhua News Agency January 11, 2009)