A man has died after being held in the back of a car by police officers in Hemel Hempstead.
The 27-year-old man died after being taken ill while being detained in Cotterells on Thursday, February 10.
Hertfordshire Police has referred itself to the Independent Office for Police Conduct (IOPC), which is now investigating the circumstances.
The IOPC says that after police detained the man at around 10pm he became unwell and an ambulance was called. However, he died at the scene.
A spokesman for the IOPC said: "After we were notified by the force, we sent investigators to the scene and post incident procedure to begin gathering information.
"Our investigation is at an early stage, but initial accounts have been provided by officers and we are gathering police body worn video and CCTV footage.
“We have made contact with the man’s family to explain our role and we will be keeping them regularly updated as our enquiries progress.”
A spokesman for Hertfordshire Police said: "The IOPC was informed of the incident, which is standard procedure when a person who has been in recent contact with the police dies.
"Our thoughts are with the man’s family at this difficult time and we will be doing everything we can to assist with the IOPC investigation to establish the exact circumstances surrounding his death.
"At this stage it would be inappropriate to comment any further."