Police have refused to comment on claims it took them 45 minutes to find a fatally injured man in the back of a van they stopped in Berkshire.
The man, in his 40s, died in London Road, Sunningdale, near Ascot, on Wednesday afternoon.
He was discovered after armed officers stopped the van and arrested four men inside it.
Police are continuing to question four men, aged 24,26, 54 and 56, being held on suspicion of murder.
The victim is expected to be formally identified later after post-mortem tests have been carried out, police said.
Thames Valley Police had been looking for the van after receiving a tip-off about a crime and it was found and stopped at about 1400 GMT.
'Found in sack'
The men, who are not from the local area, tried to escape but were arrested.
Witnesses told the BBC police appeared to hold the arrested men at gunpoint for more than 45 minutes before discovering the victim's body was in the van. They said it was contained in a "white cement sack".
A Thames Valley Police spokesman said the force would not comment on the claim.
However, Supt Richard Humphrey said: "This injured man was not readily visible from outside the vehicle and was only found when a thorough search was undertaken."
Witnesses described seeing the man's body being covered by a large blue tarpaulin, next to a white Mercedes Sprinter van.
They also said one officer at the scene remarked that he appeared to have been tortured.
Supt Humphrey said no shots were fired during the incident.
He said: "There were four occupants in the vehicle who attempted to run off. Three were detained in the immediate vicinity of the van and the fourth was arrested after a more extensive search.
"The officers returned to the van that had been stopped and then made a search of the vehicle.
"They found a seriously injured man within and immediately commenced first aid and called an ambulance.
He said despite their efforts the man died.