Detectives are questioning four suspects in connection with the death of a man who was found dying in the back of a van.
Armed officers acting on a tip-off pulled over the vehicle in Sunningdale, Berkshire, and discovered the fatally injured man inside.
When the van was stopped at about 1pm, four men tried to escape. Three were quickly arrested but officers had to use a helicopter to find and capture the fourth man.
Following the arrests, officers searched the white Mercedes Sprinter van and discovered the critically injured man, who was "breathing shallowly".
Attempts were made to revive him and at 2pm - around an hour after the van was initially stopped - an air ambulance was called. Paramedics were unable to save him and he was pronounced dead at the scene.
Eyewitnesses said the victim was wrapped in bags and one officer at the scene remarked that he appeared to have been tortured. Bystanders also said a sledgehammer, which may have been used to attack the victim, was recovered from the van by investigators.
The four arrested men are being held in police custody and questioned on suspicion of murder.
Superintendent Richard Humphrey, of Thames Valley Police, said: "At about 1pm, officers were tasked with looking for a white Mercedes Sprinter van that was believed to be travelling in the Sunningdale area of Berkshire. The officers were acting as a result of information that was given to the police, which made officers suspect that the occupants may be involved in crime."
He said that, after the suspects were caught, officers returned to investigate the van, a 1999-registered diesel model.
"They found a seriously injured man within and immediately commenced first aid and called an ambulance," said Mr Humphrey. "This injured man was not readily visible from outside the vehicle and was only found when a thorough search was undertaken. Despite the efforts of the officers and of the paramedics this man has now died."