Four suspected murders during the 1990s have been linked by South Australian police as they probe a number of criminal associations and drug dealings.

Major Crime Investigation Branch officers have concluded the disappearances and suspected murders of Leo Daly, David McWilliams, Robert Pendergast and Juan Morgan appear to be connected.

They said Juan was 15 when he disappeared in early 1992 and they believed the teenager was taken to a location, possibly in the east of regional South Australia, and shot dead, then his body buried.

Police think the teenager had stolen a car from a criminal associate and damaged property.

Of two suspects being linked to his death, one was also thought to be involved in the disappearances and suspected murders of three other victims, Mr Daly, Mr McWilliams and Mr Pendergast, police said.

Mr Daly, 33, was last seen alive in April 1998 and police said he had drug debts.

They said they thought he was assaulted by criminal associates at a house at Mitchell Park in Adelaide, then shot and thrown overboard at sea.

In late July 1998, Mr McWilliams, 39, was shot and buried, possibly also in the east of regional South Australia, police said.

Officers said the thought he was silenced after boasting about his own and others' involvement in Mr Daly's murder.

Prendergast's mother reported him missing

A drug debt is also thought to be behind Mr Pendergast's killing in 1999.

The New South Wales man, 31, was reported missing by his mother in May 1999, and she told investigators he was in Adelaide when she last spoke to him in early February of that year.

Mr Pendergast had been arrested the year before over cannabis trafficking and police said they thought he was staying with criminal associates in Adelaide, with at least two of them likely to have been involved in his murder.

Mr Pendergast is also believed to have been shot in eastern SA and his body buried.

Police said the suspects for the four presumed murders were known to them but investigators were keen to gather more evidence to support charges being laid.

Detective Superintendent Des Bray said images of the four victims had been distributed to prison inmates in the hope someone would give police some vital information.

"We know that over time loyalties change and the South Australia Police never close unsolved cases such as these," he said.

"We know that one key piece of evidence can change of the course of the investigation and lead to arrests. That breakthrough is always just one phone call away."

The cases are being investigated as part of Operation Persist, the cold case campaign by SA Police.