Brian McIlhagga, 42, was beaten and shot in both legs in Riverview Park, Ballymoney, on 5 January 2015 but later later died from his injuries.
As police issued a new appeal for information, they said they believe the UDA’s North Antrim brigade was involved in the killing.
He was attacked by a group of masked men in his home before being dragged from a house and then shot.
A 33-year-old woman who lived in the house was also beaten and four children who witnessed the attack were traumatised.
On Mr McIlhagga’s anniversary, police distributed leaflets in the area and set up checkpoints.
Det Chief Inspector Michael Harvey said “violence and brutality” in the attack “far exceeds anything I’ve experienced in 29 years of police service”.
“One would have to ask why such a level of brutality was used,” he said.
“There was certainly paramilitary involvement in this attack, but another line of inquiry is that there may well have been some sort of personal grudge.
“I believe we have identified a number of the individuals who have attacked Brian and another group of other individuals who may well have been on the periphery of this attack.”
No-one has been charged in connection with the murder and police believe that fears about paramilitary involvement have stopped people coming forward.
They have urged anyone with information to help them.
DCI Harvey added: “The family needs closure and the community needs justice.”