A motorist who was shot dead by police had been driving for an hour after being spotted pointing a handgun at a fellow driver, it has emerged.

The man travelled more than 50 miles through three police force areas before his car was stopped by armed police close to a busy motorway junction between Bristol and Portishead. A number of shots were fired and the man was declared dead at the scene. A non-police-issue firearm was recovered by officers.

It emerged on Thursday that West Mercia police received a report about a man pointing a gun at a fellow motorist at 8.30am on Wednesday near junction eight of the M5 in Worcestershire. The force passed this information on to the central motorway police group (CMPG), which covers the network in the Midlands.

At 8.40am the CMPG told Gloucestershire police about the unfolding incident. Avon and Somerset police also took calls about a man threatening other motorists and officers stopped the car when it left the M5 at junction 19 at 9.30am.

The Independent Police Complaints Commission (IPCC) is investigating the shooting, as is required by law. The road where the car was stopped, the Portbury Hundred, was closed for more than 30 hours while the scene was examined.

Officers involved are being questioned and their weapons have been seized and are being checked. CCTV footage and body cameras are also being examined. No individual police officer is under investigation at this stage, said the IPCC commissioner Cindy Butts.

The man’s identity has not been released but it is understood police know who he is.

An important part of the investigation will be to understand how the information was passed from force to force. West Mercia and Gloucestershire police both confirmed that they had received reports at about 8.30am and 8.40am of a man in a vehicle on the M5 pointing a gun at another motorist, and had passed the information on to the IPCC.

In a joint statement the chief constable of Avon and Somerset constabulary, Andy Marsh, and the area’s police and crime commissioner, Sue Mountstevens, said: “Our thoughts are with the family of the man who died and all those involved and affected. A full investigation by the Independent Police Complaints Commission is under way, and we are working to support this ongoing investigation.”

The closure of the road caused chaos in and around Portishead, including for police officers trying to get to and from their nearby headquarters.

Images of the scene showed what appeared to be a handgun on the roof of a red Suzuki Swift. There were at least four bullet holes in the offside of the vehicle and the driver’s side front window was smashed. Blood and glass, as well as a sunhat, could be seen on the floor by the hatchback.

Witnesses have given different accounts of how many shots were fired. Motorist David Ellison said: “Ahead of me were police in the road. I saw them back off from a car and then they surrounded it. They maybe shot with handguns five, six, maybe 10 times. Then they dragged a man from the car to resuscitate him.”

Frazer Phillips, from Bristol, wrote on the Hello Portishead Facebook page: “I was right next to it when it happened and heard about 4-5 shots.”

The IPCC confirmed it was looking at the West Mercia report.

An IPCC spokesperson said: “We have received a referral from West Mercia police in relation to a report they received from a member of the public prior to the incident near Portishead. We are assessing the referral and whether we should also investigate West Mercia police actions.” It has not commented on Gloucestershire’s involvement.