An incident where a man was critically injured after falling from a moving ambulance while in the company of a police officer is being investigated.
A 46-year-old man was taken to hospital after the fall in Exchange Road, Watford, on Monday at about 10:30 BST.
The Independent Office for Police Conduct (IOPC) said it would "look at the circumstances of the incident and the actions of police prior to it".
Hertfordshire Police confirmed it referred itself to the IOPC.
A spokesman for the IOPC said: "We can confirm we have begun an independent investigation into the actions of the police concerning an incident in Watford, Hertfordshire, in which a 46-year-old man was injured and is currently critically ill in hospital."
The IOPC said it had taken initial accounts from the police and would be liaising with the man's family to explain its role.
It appealed for any witnesses to contact the organisation.