Detectives think a 17-year-old boy died after taking a course of muscle-building drugs.
The teenager - named by friends as Matthew Dear of Southend, Essex - died at Southend University Hospital on Monday.
Police said he had been undergoing treatment since April 13 and it was thought that he had become seriously ill after taking anabolic steroids.
Detectives said tests were being carried out to establish what tablets the teenager had taken.
A police spokesman said a post-mortem examination was due to be carried out.
He said three people - a 17-year-old boy and a 21-year-old man, from Westcliff, Essex, and a 24-year-old man from Leigh, Essex - had been arrested on suspicion of supplying a controlled substance. They were all released on police bail without charge until June 23.
Several people paid tribute to the teenager in emails to a local newspaper website.
One family friend wrote to say that Matthew wanted to be a Royal Marine and fight in Afghanistan. She said: "We saw him grow from a bonny baby boy to a lovely caring young man... he wanted to become a marine ...He had a dream, to one day become a soldier."
Anabolic steroids are usually synthesised from the male reproduction hormone testosterone. They reduce fatigue and recovery time and promote the development of muscle tissue by stimulating protein.
The drugs are banned by many sporting bodies because they create a health risk and can give athletes a competitive advantage.
