Police have identified the bones found in a septic tank at the back of a property in Brisbane's north last month as the remains of Wayne James Youngkin.
Mr Youngkin is believed to have been aged 29 or 30 when he went missing in 1986, police said.
The man had lived at the 136 North Road address in Brighton, where his remains were found by police on November 29, with his grandmother and uncle prior to his disappearance.
Detective Superintendent Mick O'Dowd said police were treating Mr Youngkin's death as a murder investigation.
"(It was) Suspicious from the outset and we have treated it as a murder investigation since that time," he said.
"The cause of death still to be determined."
Mr Youngkin's body was found in the decommissioned septic tank along with a gold watch, a ring and a number of items of clothing that police used, along with dental records, to identify the missing man.
"Police discovered a ring and a gold watch that had the initials of deceased," Superintendent O'Dowd said.
"During the investigation, we found a photograph of the deceased who was wearing a number of items of clothing similar to items of clothing that were discovered during an examination of the pit where the deceased was located."
The last known activity of Mr Youngkin was when he applied for a passport in 1986.
"From that we haven't been able to identify any movement or activity of the deceased from then," Superintendent O'Dowd said.
Police are eager to speak with a man who called Sandgate police station on August 22, 1992 in relation to Mr Youngkin's disappearance.
"(The caller) was quite specific in relation to information they gave us to what they believe may have been the cause of death of Mr Youngkin," Superintendent O'Dowd said.
"From the last known time we have of his movements in relation to his passport signature and 1992 is a number of years again so in that time they have got some information that they thought important enough to call police and give us that information."
"We are urging that person to make contact with us again, either through Crime Stoppers or Carseldine CIB so we can discuss that information at length with them that they supplied to police."
The deceased's uncle and grandmother, who lived at the North Road address until Autism Queensland bought the property 30 years ago, have since died.
Superintendent O'Dowd said he believes Mr Youngkin died in 1986 "or thereabouts".
Police are calling on anyone who knows what happened to My Youngkin to contact Crime Stoppers on 1800 333 000.