The police watchdog has begun an investigation following the death of a man who was stopped in a car by armed officers.
Leicestershire Police officers, who were investigating a murder, followed the 46-year-old driver on the A52 Bingham bypass near Nottingham early on Thursday.
The unnamed motorist suffered serious head injuries and was given first aid but was pronounced dead at the scene, and a non-police issue firearm was discovered in the silver Mazda Tamura.
Police were investigating after the body of a man in his forties was found in a field in Plungar on Tuesday afternoon.
A 56-year-old man from Grantham was arrested on suspicion of murder on Wednesday and has since been released under investigation.
The force has made a mandatory referral to the Independent Office for Police Conduct (IOPC) and said it will cooperate fully.
An IOPC spokesperson said: "Early today we were notified by Leicestershire Police about an incident at around 12.30am on the A52 Bingham bypass in Nottinghamshire where armed officers involved in a murder investigation had followed a man driving a silver coloured Mazda Tamura.
"We were advised that, after making the vehicle stop, the officers found the 46-year-old driver had a serious head injury and gave immediate first aid.
"Sadly, he was pronounced dead at the scene. A non-police issue firearm was also discovered in the vehicle.
"The coroner has been informed."
The IPOC announced an independent investigation will consider the circumstances of the police involvement.
"Initial accounts have been provided by officers involved and we have also gathered police body-worn footage for review," the IOPC spokesperson added.
"Our thoughts are with the family and friends of the man who died, and all those affected by this incident."
Road closures remain in place on the A52 Bingham bypass, the police said.
The senior investigating officer in the murder investigation, Detective Inspector David Greenhalgh from the East Midlands Special Operations Unit, said: "We do continue to appeal to anyone who was in the Plungar area on Tuesday afternoon or who has any information in relation to the incident to come forward to us.
"Did you see anyone go into a field? Did you see or hear anything unusual or which caused you concern?
"I understand there will be concern in the community and our officers remain in the area offering reassurance as our enquiries continue. Please do speak with them."