Queensland detectives believe they have reached a turning point in the investigation of a triple murder in Brisbane more than 40 years ago.

A unnamed man may hold the key to an unsolved triple-murder case 40 years ago, which could be linked to Brisbane’s underworld.

Barbara McCulkin and her daughters Vicki Maree, 13, and Barbara Leanne, 11, disappeared from their Highgate Hill home in January 1974. Their bodies have never been found.

On Tuesday, police from the Queensland Crime Command Homicide Cold Case Unit searched two rural properties on the Southern Downs in Warwick, southwest of Brisbane.

Detective Superintendent Steve Holahan said the raids turned up items of significance.

“I’m led to believe that a number of items of property corroborate different information and versions that have been provided to us by witnesses,” he told reporters in Brisbane.

The reinvigorated investigation has led detectives to interview up to 30 witnesses.

Det Supt Holahan said documents were seized from the homes, but would not elaborate on what other “physical items” were found there.

The properties are associated with one of two principal persons of interest, but no warrant has been issued for his arrest.

Police will make inquiries to locate the unnamed man.

“It’s up to him whether he speaks to us or not,” Det Supt Holahan said.

There have been suggestions Mrs McCulkin had information about the Whiskey Au Go Go nightclub firebombing in Brisbane 10 months before her disappearance and was going to turn police informant.

Det Supt Holahan confirmed police are examining the suggestion Mrs McCulkin had knowledge of “serious crimes” as a motive for her murder.

There is also speculation her daughters had been sexually assaulted.

“We’re looking at both of those motives,” Det Supt Holahan said.

Detectives are also looking at a number of other crimes committed around the same time by “an organised group of people and their associates”.

Det Sup Holahan said police had not ruled out the possibility that other people might have been involved.

In 1980, a Brisbane coroner recommended two men be charged over the three murders.

The following year, the crown abandoned the charges because there wasn’t enough evidence to convict the men.

A reward of $250,000 for information that leads to the arrest and conviction of the person or persons who murdered the trip has been offered.