Andrew Burns was shot dead at Donnyloop in Castlefin, Co Donegal, on February 12, 2008 after being lured there from his Strabane home.
The 27-year-old received a number of gunshot wounds and was later found on a road close to Donnyloop Church.
He had been shot in the back and was pronounced dead shortly before 8pm.
The car used to transport Burns to the murder scene was later found burned at Coshquinn, outside Derry city.
Dissident group Oglaigh na hEireann was blamed for the murder.
One person has been convicted of his murder. But gardai say they believe that a number of people were involved, and highlighted how relationships and loyalties may have changed in the time since the murder.
Detective Inspector Pat O’Donnell said: “An Garda Siochana in Donegal would like to directly appeal to those individuals who were on the periphery of the group involved in Andrew’s murder.
“In particular their friends, family members and partners.
“It has been 14 years since Andrew’s murder and during that time relationships change, loyalties change and people may not now be constrained by the set of circumstances that existed for them some 14 years ago.
“An Garda Siochana is appealing to those individuals to look into their conscience and unburden themselves of whatever information they may have.”
Gardai said they are grateful for all the witnesses who have come forward and made statements in the intervening years, but it is never too late for those who have yet to come forward to make contact.
Up to 12 people are believed to have been involved, but only Martin Kelly (47) from Strabane was jailed for life in 2012 for his part in the murder.
Kelly was told to bring Mr Burns from his home in Strabane to Donnyloop for a punishment shooting. Three men were waiting in a hedge at the church car park.
A gunman fired at Mr Burns, who ran before being killed by a second shot. The three men then got into Kelly’s car and he drove them back across the border, stopping to hide the gun.
Information will be treated in the strictest confidence, gardai said.
Anyone with information is asked to contact the incident room at Letterkenny Garda Station on 074 9167100, or the garda confidential line 1800 666 111.Tags: