Thames Valley Police is investigating after gunshots were fired from a car in Aylesbury.
Officers were called to an incident in Church Square, during which it is believed a gun was used.
There were no injuries following the firearm discharge.
Following this, both vehicles drove away, and later collided with one another in nearby Hampden Gardens.
A 26-year-old man was then assaulted, resulting in him being taken to hospital with critical injuries, where he remains.
The firearm is not believed to have been used during the second incident and officers are attempting to locate it.
A 54-year-old man has been arrested on suspicion of attempted murder in connection with this incident and is in police custody.
Police are seeking to identify two further people who are believed to have been occupants in one of the vehicles.
Road closures are in place in Church Square, Hampden Gardens, Churchill Avenue, Carrington Road, and Chaloner Road. A scene watch also remains in place while officers carry out enquiries and examine the scene.