Christopher Guest More Jr. fled the UK after the murder of Brian Waters, who was subjected to 'systematic and barbaric torture' as his family watched helplessly at Burnt House Farm in Tabley, near Knutsford in England in June 2003, the Daily Mail reported.
More, 41, who described himself an undercover journalist, has links to Spain, Malta and South America, but detectives believe he 'could be anywhere.'
Former undercover BBC researcher James Raven, 60, Otis Matthews, 41, and John Wilson, 69, are serving life sentences after being convicted of Waters' murder.
More is alleged to have been with the men when they stormed the derelict property, where Waters was running a cannabis farm, in a row over a drugs debt.
The men tied Waters to a chair before battering him in front of his son Gavin, who was also attacked, and daughter Natalie, who had just turned 21 and was held at gunpoint and forced to watch.
The 44-year-old victim was whipped, burned and attacked with a staple gun in a three-hour ordeal during which he sustained 167 injuries, a trial heard.
He was then hung upside down and assaulted with an iron bar, causing fatal internal injuries.
More, who is white and six feet tall, is wanted for questioning in connection with Waters' death along with the attempted murder of a second man and the false imprisonment and assault of other victims at the scene.
On Tuesday Cheshire Police revealed he had been added to Europe's most wanted list.
More's cousin Raven worked on programs including the BBC's MacIntyre Uncovered and Crooked Britain, as well as Channel 4's Sleepers, infiltrating gangs involved in car crime, drugs and counterfeiting.
A Chester Crown Court trial in 2004 heard More also bragged about work as a surveillance-type journalist and had been following Waters' son nine days before the murder in what he described as a 'reconnaissance' mission.
After the systematic torture in the cow shed at the farm, Waters' body was dumped in a milking parlor.
Waters, a former chemists' worker, had rented Burnt House Farm since 2002 and used it to grow cannabis, a court heard.
But he was also involved in narcotics trafficking with one of his killers, drug dealer John Wilson, and was said to have owed him £20,000.