A section of the M40 remains closed as police carry out searches where a motorcyclist was shot dead.
The murder victim, aged in his 30s, died on the southbound carriageway near Leamington Spa in Warwickshire.
Biker gangs are a "strong line of inquiry" said police, who also want to trace the occupants of a green Rover 620 seen near where the man was shot.
The motorway reopened northbound on Monday afternoon but remained shut between junctions 14 and 12 southbound.
About 50 specialist officers were conducting a fingertip search of the scene.
The man, who died between junctions 13 and 12, is thought to have attended the Bulldog Bash, billed as Europe's top biker party, held at Long Marston airfield near Stratford.
Witnesses saw car
A post-mortem examination is to be carried out on the man's body later on Monday.
Police have said they do not believe it was a road rage attack.
The shooting bears similarities with a similar incident in 2001 when a Canadian man, who had spent the weekend at the Bulldog Bash, was shot in the leg as he rode his motorbike down the M40 towards London.
About 400 vehicles were trapped at the crime scene (pic Ben East)
Det Supt Ken Lawrence, of Warwickshire Police, said officers wanted to speak to people travelling in the Rover 620 who may have been linked to Sunday's shooting.
"One or two witness accounts have mentioned this car being close to the motorcycles at the time something clearly happened," he said.
"It is for that reason I want to trace that car and speak to the occupants."
He said detectives were not aware of any disputes at the Bulldog Bash which could be linked to the shooting.
He added: "If there were any we are anxious to find out what they were and what they were about."
The incident caused 10 mile (16km) tailbacks in both directions with about 400 cars caught within the crime scene. They were all allowed off the motorway by Sunday night.
Paul Roberts, from Worthing, was driving along the M40 when the incident happened.
He said: "Suddenly we had to stop... soon other motorists were out of their cars and on the carriageway.
"One lady in a BMW took a first aid kit to the man who was lying on his front just next to the central reservation."
Police are appealing for any motorists travelling on the M40 who may have witnessed the incident to contact them.
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