A driving agency boss attacked at work was killed by "pressure on his neck", say Gwent Police.
Company director Kingsley Monk, 45, died at the Driverline 247 offices in New Inn, near Pontypool, on Monday.
Detectives believe his killer lay in wait for him to arrive at the offices before restraining him and three colleagues, with cable ties.
A man aged 52 is still being questioned by officers on suspicion of murder, attempted murder and false imprisonment
The four men held at Driverline 247 were assaulted, threatened with a gun and doused in a mystery substance in a four hour ordeal, police said.
The unit, on an industrial park, was then set alight with the men inside.
Silver handgun
Businessman Allan Styles rescued three of the men but Mr Monk, 45, from Oakdale near Blackwood, was already dead.
The three other men were treated at the Royal Gwent Hospital in Newport but later released.
A Gwent Police spokeswoman said on Thursday: "Officers are now able to confirm the preliminary findings of the post-mortem examination, which shows that the death of Kingsley Monk was in keeping with pressure on his neck."
Police are still searching for a silver handgun which they believe the attacker discarded along the A4042 near Newport.
Officers have also appealed for information about an RBS black credit card reader used in the incident, a black laptop bag and a long-sleeved blue shirt.
Detectives, who have said they are not looking for anyone else in connection with Mr Monk's death, were granted an extra 36 hours on Wednesday to question the Cardiff man arrested at Newport railway station on Monday evening.
Police made their arrest after raiding an address at Penrhos Crescent, Rumney in Cardiff.
The property was surrounded for six hours by armed officers before it was found to be empty.
Neighbours said the man who lived in the house was Russell Carter.
The identity of the man who was arrested has not been released.
Anyone with information on the items sought by police is urged to contact Gwent Police on 01633 83811 or Crimestoppers on 0800 555 111.
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