COPENHAGEN, Denmark (AP) — At least 27 people were hospitalized after being poisoned at a rave party in Oslo by carbon monoxide given off by portable generators, Norwegian media reported. Police said Monday that two people faced preliminary charges of trespassing and illegally being inside a bunker.

Five people were hospitalized in critical condition — including two police officers who were the first at the scene — 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.

More people could face preliminary charges, police said, adding that the weekend rave is still being investigated. Preliminary charges are a step short of formal charges.

Up to 200 party-goers in their 20s and 30s had gathered in the bunker in the Norwegian capital 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 more people unconscious in the bunker.

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 insisted that it did not bear any responsibility, Norway’s VG newspaper reported. The entrance to the bunker had been previously closed with double-reinforced concrete but the new owners only secured it with wooden boards.