Four building maintenance workers were killed Thursday in a basement parking garage in Tokyo as a fire suppression system was unintentionally activated, trapping them in an enclosed space with high levels of carbon dioxide, police and firefighters said.

A total of six men were replacing ceiling linings in the parking garage of a condominium building in Shinjuku Ward when the fire suppression system caused shutters to close and began discharging CO2 as a fire extinguishing agent, the police said.

The four men ranged in age from their 30s to their 50s, the authorities said, adding that one worker in his 20s is in critical condition and receiving treatment at a hospital.

Rescue workers measured the CO2 density in the parking garage as about 20 percent, several hundred times higher than the normal level, the Tokyo Fire Department said.

Source: ANNnewsCH