Police believe two young girls were sexually assaulted before they were murdered along with their mother in a 40-year-old crime that could be linked to Brisbane’s underworld.

Police have searched two rural properties in the Warwick area, southwest of Brisbane, after receiving new evidence regarding the fate of Barbara McCulkin and her daughters Vicki Maree, 13, and Barbara Leanne, 11.

They were last seen at their Highgate Hill home in Brisbane’s inner south on January 16, 1974.

Over the years, there’s been speculation Mrs McCulkin had information about the March 1973 firebombing of the Whiskey Au Go Go nightclub in Brisbane, in which 15 people died, and the trio was killed because she was going to be an informant.

Police said on Tuesday they had compelling new evidence about the crime after Mrs McCulkin’s family made an emotional appeal for information in January, on the 40th anniversary of the disappearances.

They said possible motives had expanded beyond suggested links to organised crime to also include a sexual element.

“Certainly there was a suggestion Barbara McCulkin had knowledge of serious crimes committed by a number of persons,” Detective Superintendent Steve Holahan told reporters.

“There’s also information that has come to light that suggest that her daughters were sexually assaulted as part of this crime.”

He did not answer questions about possible links to the Whiskey Au Go Go attack.

In 1980, a coroner ruled that two men be charged with the murders. One of those two men was known to Mrs McCulkin’s estranged husband, Billy McCulkin, who had worked as a debt collector.

But in 1981, the charges against the two men were dropped on the basis there was insufficient evidence to secure convictions.

Police say their recent review of the case has unearthed compelling new evidence, including new witnesses, about events on the day the trio vanished.

But they say their investigations continue to focus on information and persons of interest identified in the original investigation and 1980 inquest.

The properties searched near Warwick are linked to one of the two principal people of interest in the case.

Det Supt Holahan said police were trying to find the unnamed man, who was not at home during the raids. But he said no arrest warrant had been issued.

“We’ll look to speak to him further in relation to this matter and the items that have been located at his property. It’s up to him whether he speaks to us or not,” he said.

Det Supt Holahan said seized items included documents that supported new evidence from witnesses. Other physical evidence was also seized, but he did not say what.

“Certainly some of the items that I’ve been briefed on that have been located corroborate the new information we’ve got,” he said.

He said he hoped Mrs McCulkin’s family would ultimately achieve justice for her and her children.

“It’s one of those things that is a very distasteful crime. We’re very hopeful of a positive outcome.”