An 18-year-old man died after taking drugs at a music festival, police have said.
Christian Pay, of Millom, was among five people taken ill at Kendal Calling in Cumbria on Friday.
Police said three different types of tablet could be linked to his death and warned festival-goers not to take illegal drugs.
Four people have been arrested on suspicion of possession with intent to supply, Cumbria Police confirmed.
Mr Pay was pronounced dead after being admitted to the Cumberland Infirmary, where a further four people have been admitted.
Police said a 19-year-old man from Millom and a 29-year-old woman from Bolton were in a "critical condition" and two men - aged 18 and 20 and both from Millom - were "serious but stable".
According to police, drugs seized included a blue tablet with an Adidas logo, a yellow powder believed to be MDMA, an orange oval tablet with a Rolls Royce logo, a green tablet with a white fleck and a plain grey tablet with a white fleck.
Supt Mark Pannone said: "These are just some of the drugs seized. Our advice is simple, do not take any drugs at Kendal Calling 2015.
"It is never safe to take unknown substances as you cannot be sure of what they contain.
"There is a risk that these substances are on still on site and those attending the festival need to be aware of the potential dangers of taking substances such as these."
A spokesman for the festival said organisers were "deeply saddened" by Mr Pay's death.
"Our thoughts and condolences are with his family and we are doing everything possible to support the police," he said.
The North West Ambulance Service said five ambulances attended the scene at Penrith at about 06:42 BST.
The festival runs until Sunday.