Three other people have been treated in hospital.
Oxford: Ecstasy Warning After Teen Dies
The family of a 15-year-old girl who died following a heart attack after it's believed she took ecstasy have spoken of their devastation.
Martha Fernback fell ill at Hinksey Park in Oxford on Saturday.
Relatives of the teenager, from Oxford, described her as a ``beautiful girl'' whose sudden death had left them ``utterly devastated''.
In a statement issued via Thames Valley Police today, her family said: '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.'
Officers who are treating Martha's death as 'unexplained' attended the scene and she was taken to the John Radcliffe Hospital, but later died.
A post-mortem examination will be conducted today (Monday 22nd July) but the results of toxicology and histology tests are not likely to be known for several weeks.
A 17-year-old boy from the Oxford area has been arrested on suspicion of possession with intent to supply a controlled drug in connection with the inquiry.
The death of Martha has prompted detectives to issue a warning to young people to avoid dabbling with illegal drugs such as MDMA.
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 inquiries 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.
'A full post-mortem examination will be carried out tomorrow, which will include toxicology and histology tests. It is likely to be several weeks before the results of these tests are known.
'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.'
Tributes to Martha were paid on Twitter. One wrote: 'r.i.p martha fernback. you will never be forgotten, you have so many people that love you. rest in peace angel.'
Another tweeted: 'R.I.P Martha Fernback heaven has gained a new angel!' On Facebook, a page dedicated to her memory had gained 1,300 `likes'.
A message posted on the page read: 'Her cause of death is under investigation. We are all devastated and in shock.'
One message of condolence on the page read: `'Such a special creature, such an ever-sparkling little thing.' Another poster wrote: 'Martha was a beautiful young soul stolen from us for heaven.'
Anyone with information is asked to call Thames Valley Police on 101, or Crimestoppers anonymously on 0800 555 111.
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