A teenage girl died in front of horrified sunbathers as paramedics tried to revive her after she had taken a tablet she believed was MDMA.
Martha Fernback, 15, was airlifted from Hinksey Park in Oxford to John Radcliffe Hospital but was pronounced dead on arrival.
Senior investigating officer Detective Inspector Brendan Murray said: "Firstly our thoughts are very much with Martha's family, who have been left devastated and distraught by her death.
"Following our initial enquiries we now understand Martha had taken a substance earlier that day which she believed was MDMA.
"It would be wrong to speculate on what this substance was or what caused her death, as the investigation is still at an early stage."
He added: "I would strongly urge young people not to take or experiment with illegal drugs such as MDMA. They are extremely dangerous and often contain poisons and illicit chemicals which can prove fatal."
On Sunday, police arrested a 17-year-old in Oxford on suspicion of possession with intent to supply a controlled drug.
Speaking to the Telegraph anonymously, a friend said Fernback had taken a PMA tablet, which is similar to MDMA and is often sold as ecstasy.
PMA, also known as Dr Death, is stronger than ecstasy though, and can cause a fatal increase in body temperature.
It also takes longer to take effect than MDMA, sometimes leading users to take several in succession in the belief that they are not working.
It is sold in pink tablets for as little as £3.
A post mortem is due to take place on Monday, but it could be weeks before the results of histology and toxicology tests become known.
The family said in a statement: "Our beautiful girl was taken away from us yesterday, she was the most extraordinary, effervescent human being and all her family and friends felt so privileged to have known her.
"We are all utterly devastated and have sought such comfort in the beautiful condolences we have received from everyone."
A Facebook page set up in memory of Fernback already contains hundreds of messages of condolence from friends and well-wishers, and many were arranging to meet in Hinkley Park on Sunday to remember her.
Amanda Tracey wrote: "Still crying since last night. Went to bed crying and woken up crying. I only had the privilege to meet Martha a few times but when i did she was such a beautiful cheeky character :-). With great curly hair!"
Kirsten Armstrong added: "'Earth overcome grants you the stars'. Rest in peace, beautiful Martha. You will be sorely missed."
Fernback's mother, Anne-Marie Cockburn wrote: "Thanks so much to all Martha's friends, I am finding so much comfort in reading your lovely messages about my girl. We all loved her so so much. If you have any photos, videos etc, please, please post them up and I will use them. Xx"
PMA has been linked to a number of deaths of young people in Britain in recent months, as well as deaths in Canada, Norway and Denmark.
Last week, an inquest found that 19-year-old gym instructor Travis Barber of Salford, Greater Manchester, had died after taking the drug.