The body of Jeff Mundy, who was 36 when he went missing from Adelaide in December 2020, was discovered on a walking track on the Fleurieu Peninsula on April 24.
SA Major Crime chief Detective Superintendent Darren Fielke said police were offering a $200,000 reward for anyone who could provide information that assisted in the conviction of Mr Mundy's killer or killers.
"We believe there are people who know what happened to Jeff," he said on Friday.
"Over the last five years, people who did not come forward, their circumstances and their allegiances may have changed - now is the time to come forward."
A bushwalker found Mr Mundy's decomposed body five metres down an embankment, off the side of a track off Mount Alma Road at Inman Valley.
His parents, Rob and Belinda Mundy, said they could "never express enough gratitude" to that person for helping to bring their son home.
"For five-and-a-half years, our family has lived with unbearable ache, the pain of not knowing what happened to Jeff or where he was," Mr Mundy said.
"Jeffrey was a father, a loving son, a brother and an uncle, and he was taken from us in the most horrific circumstances."
The suffering and heartbreak had impacted their entire family and was something "we are forced to live with each and every day", Mr Mundy said.
"We know there are people out there who know what happened to Jeff, and who are responsible for taking his life," he said.
"If anyone knows anything at all, come forward with your information, and that could be the missing piece that helps bring those responsible to justice for our family."
Mr Mundy was last seen by his family on December 19, 2020, in the southern suburb of Seaford.
In 2021, police launched Task Force Southern to investigate the deaths of Mr Mundy and two other men, Robert Atkins and Trevor King.
At the time, police said they believed between 12 and 15 people, described as "low-level drug dealers and petty criminals", were involved in the murders.
But Supt Fielke said police had established Mr Mundy's disappearance and murder were not connected to the Task Force Southern investigation, which had since been disbanded.
"We will be reviewing all aspects of the initial investigation, including how many people were involved in both Jeff's murder and protecting those people who were responsible for his murder," he said.
Police also released CCTV footage of Mr Mundy's last known movements on the day he disappeared.
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