In the French department of Cher, dozens of attendees were injured during an illegal rave party: 12 people were hospitalized, including some in serious condition; this is believed to be linked to drug use, France Info reports.

The incident occurred on Sunday, May 3, near the city of Bourges. According to the prefecture, about 17,000 people gathered on the grounds of a former military training site, which authorities consider dangerous because of the possible presence of unexploded ordnance.

According to French Interior Minister Laurent Nuñez, a total of 28 people sought medical assistance in a condition of relative urgency, and 5 were in serious, critical condition.

The main cause of the hospitalizations, according to preliminary information, was drug use during the event.

The event was an unauthorized “free party,” that is, an illegal music rave. Authorities said it was organized on the grounds of a former military facility, which remains potentially dangerous because of unexploded shells left over from the war.

Interior Minister Laurent Nuñez, who arrived at the scene, called what was happening an “illegal musical event” and said that the participants had deliberately chosen this site. He also noted that after the rave ends, everyone leaving the area will be fined — both for illegally entering a military site and for taking part in a prohibited mass gathering.

More than 600 gendarmes were mobilized to control the situation, and 14 checkpoints were set up around the rave site.

Authorities were also further concerned by the discovery of unexploded ordnance. On Saturday, a shell was found near the RD102 road, after which bomb disposal experts moved it to a safe place. Later, another piece of ordnance was discovered, and work to neutralize it is ongoing.