Some 25 people taking part in an illegal rave in a bunker in the Norwegian capital, Oslo, were poisoned by carbon monoxide given off by portable generators, police said.
Five people - including two police officers who were first on the scene - were hospitalized in critical condition but their lives are not in danger, the Norwegian news agency NTB said. Two of the five were released from intensive care on Monday, NTB reported.
Up to 200 partygoers in their 20s and 30s had gathered for the rave, which used portable diesel generators to power lighting and sound systems.
Officers discovered the event early Sunday when a police patrol met a group of confused young people in the park where the bunkers lies. Emergency services found seven people unconscious in the bunker.
Norwegian broadcaster NRK said Sunday no had yet been accused of wrongdoing.
The party started late Saturday night and had been announced on closed social media pages.
The company that owns the bunker described the illegal rave as a "serious break-in'' and said the company did not bear any responsibility, Norway's VG newspaper reported. The entrance to the bunker was previously closed with double-reinforced concrete. However, it was reopened by new owners and only secured with wooden boards.
The organizers later said the diesel generators were in a room with ventilation but neither police nor the fire department could confirm that. Several partygoers told Norwegian media that they had to go outside several times to breathe fresh air.
Carbon monoxide is an odorless, colorless gas produced by the burning of carbon-based fuels, including gas, oil, wood and coal. The gas is harmful because it displaces oxygen from red blood cells, resulting in damage to major organs.
Exposure can cause headache, dizziness, nausea, coughing, breathing problems and irritation of the eyes, nose, and throat, and in some cases it can be fatal.