A man who died after being found seriously injured in the back of a van stopped by armed police had suffered head injuries, tests have revealed.
The man, in his 40s, was found when the vehicle was pulled over in London Road, Sunningdale, Berkshire, on Wednesday. Four men inside were arrested.
Thames Valley Police refused to comment on claims it took them 45 minutes to find the victim after stopping the van.
It said it would inform the Independent Police Complaints Commission on Friday.
Meanwhile police are carrying out a forensic search of a property in Hancocks Mount in Sunningdale in connection with the incident.
Detectives have also been granted an extra 36 hours to question the four men in custody, aged 24, 26, 54 and 56, who were all arrested on suspicion of murder.
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.
The victim, who is due to be formally identified on Friday, was seriously injured when he was found, but died at the scene, police said.
The four men, who are not from the local area, tried to escape but were arrested.
A post-mortem examination carried out on Thursday revealed the victim died of head injuries.
Eyewitnesses told BBC News police appeared to hold the arrested men at gunpoint for more than 45 minutes before discovering the injured man in the van. They said it was contained in a "white cement sack".
A Thames Valley Police spokesman 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.
Mr Humphrey said no shots were fired during the incident.