A 14-year-old girl has died after taking an "unknown substance".
She collapsed in Camborne, Cornwall, at about 22:40 BST on Friday and was pronounced dead in hospital on Saturday morning, officers said.
A 15-year-old boy and two 15-year-old girls were also taken to hospital after taking the substance. One of them is said to be in a serious condition.
Two teenage boys have been arrested on suspicion of being concerned in the supply of a controlled drug.
They are a 17-year-old boy from Redruth and a 15-year-old boy from Camborne, Devon and Cornwall Police said.
Officers said the unknown substance had been seized and would be tested.
Det Supt Roy Linden said it was believed all four were "hanging around a scrapyard" before heading to the home of one of the teenager's.
"All four have then gone on to Pengegon Park and an area known as The Courts, which is a well-known area for teenagers to gather," he said.
"A short time later the 14-year-old girl collapsed and an ambulance was called.
"This is a truly tragic incident in which a young girl has lost her life.
"Safeguarding young people is a priority and we ask parents and carers to speak to their children about the risks associated with taking drugs".
The families of all the children are being supported and the force is appealing for information, Mr Linden said.