Devon and Cornwall Police have referred itself to the Independent Police Complaints Commission (IPCC).
The force said the two suspects and victim had recently come to the attention of police for an earlier incident but could not go into more detail.
More top news
Leah Kerry, 15, died after taking a new psychoactive substance in a park in Newton Abbot. Her family have spoken about their grief.
It was a busy weekend in Bristol as crowds gathered for the city's annual Harbour Festival featuring boats, food and musical performances.
Dry and bright for much of the time, rain on Wednesday