The police complaints watchdog is to investigate the level of force used by officers after a man's finger was severed while he was being detained.
The Independent Police Complaints Commission (IPCC) said the 43-year-old man was arrested in Dronfield, near Chesterfield, last month and placed in a caged area in the back of a van.
But officers from Derbyshire Police only realised his left hand was bleeding when they arrived back at Chesterfield police station.
It is not clear at what stage the man was injured during the incident, which occurred late on June 6.
The man's little finger was recovered but despite treatment at hospital the digit was unable to be reattached.
In a statement, the IPCC said: "The IPCC investigation is looking at the circumstances surrounding police contact with the man, including his arrest and the appropriateness of force used.
"Investigators are also examining how the man was placed in the rear of the police van, whether any observations were carried out whilst in police transit and whether any medical treatment was provided before being taken to hospital."
Anyone who witnessed the man being placed in the police van at around 11.40pm near the Green Dragon pub in Church Street is urged to contact the IPCC on 0800 151 0020 or email dronfield@ipcc.gsi.gov.uk.