Two 19-year-old men have died in hospital in the early hours after apparently taking drugs at a nightclub.
They were found unconscious at the Pryzm club in Plymouth, where hundreds of young people were attending a gig by the Swedish dance artist Basshunter.
Police said the teenagers, from Okehampton and Newton Abbot, were thought to have taken MDMA.
The club was evacuated and an 18-year-old man was arrested by Devon and Cornwall Police.
At about 02:00 GMT the poorly men were taken to Plymouth’s Derriford Hospital, where they were later pronounced dead.
Det Insp Julie Scoles said the two who died were part of a larger group who took the drug.
“We have located the rest of the group who are thankfully showing no ill-effects at this time,” she said.
“I am urging the public, especially those going out and planning to take recreational drugs, to be aware of this incident and think twice before taking any unknown substance – there is always a risk when taking drugs and the only way of staying safe is to avoid drugs altogether.”
Next-of-kin have been informed, but formal identification of the victims has yet to take place and police have asked anyone with any information to contact them.
The nightclub said the deaths were “tragic and very sad”, and staff were co-operating with the police investigation.
A statement said: “We are devastated by the events of this morning where two young men tragically lost their lives, and we would like to extend our thoughts and condolences to the families and friends at this very sad time.”