Andrew Burns was shot twice in the back in a church car park in Co Donegal.
It happened around 7pm on Tuesday, February 12, 2008 at Donnyloop, Castlefin.
Mr Burns was later found on a roadway close to Donnyloop Church.
The 27-year-old, who was believed to have been a member of the Real IRA, 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 republican terror gang Oglaigh na hEireann (ONH) were blamed for the murder.
Gardai have previously said they believe 12 people were involved.
But just one man was convicted in relation to the killing. Martin Kelly, also from Strabane, was jailed for life in 2012 for his part in the murder.
In a new appeal, gardai said Mr Burns’ family remain devastated by this callous act and continue to struggle to come to terms with his 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 13 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 13 years ago.
“An Garda Síochána is appealing to those individuals to look into their conscience and unburden themselves of whatever information they may have.”
He said detectives are grateful for all the witnesses that have come forward and made statements in the intervening years, but said it is never too late for those who have yet to come forward to make contact with police.
He added that information received by gardai will be treated in the strictest confidence.
In 2018, on the 10th anniversary of Mr Burns’ death, his sister Jacqueline issued an emotional plea for information.
She said: “It has been a journey that you learn to live with, you never forget.
“Andrew did not deserve what he got. It was well over the top.
“Their mindset, their way of thinking, the location of the murder, on sacred ground within a church grounds, went against everything that is humanly decent.
“My brother Andrew was left at the side of the road to die.”
Anyone with information should contact the incident room at Letterkenny Garda Station on 074 9167100, the Garda Confidential Line 1800 666 111 or make contact with any Garda Station.Tags: