The Independent Police Complaints Commission's started an investigation into how a teenager, arrested last month, ended up in intensive care.
The 18 year old was arrested by police in Southend at around 3pm on Saturday September 13th following a police pursuit.
After being taken to Chelmsford Police Station the man complained of severe abdominal pain in the early hours of Sunday and was taken to hospital where his condition is said to be critical but stable.
The IPCC received a complaint from the man's sister a week later and investigators are now looking at footage of what happened, as well as speaking to independent witnesses and examining officers' initial statements.
Anyone who witnessed the arrest is asked to contact the IPCC on 020 7166 3000.