But firefighter Tom Day, the father of 19-year-old Darcy, was warned to stop looking for answers.

"I remember dad saying what was told to him, 'Give up, Tom, you're getting too close,'" Darcy's sister Dianna Day said in a statement read on Monday to an inquest into the 1973 arson attack that killed 15 people.

While their mother attended every court hearing, Mr Day made it is mission to find out who was behind the bombing of the Brisbane nightclub.

Until he was warned to stop.

"Fearing for his safety and his family's, he stopped looking," Ms Day said.

The inquest into the firebombing re-opened for a two-week sitting in Brisbane on Monday - some 48 years after the first three-day inquest ended when two men were arrested.

Coroner Terry Ryan is set to determine whether the two - James Richard Finch and John Andrew Stuart - who were convicted and sentenced to life over the attack were the only people who caused or contributed to the deaths.

Counsel assisting the coroner Stephen Keim asked in his opening address whether there was "some evidence at least" that indicated whether Finch and Stuart may not have been the only people responsible.

"The answer to that question is that such evidence does exist," he said.

Mr Keim said there was considerable evidence that people associated with the nightclub knew an attack was pending and "in some cases warned acquaintances not to be present".

There were also several suspicious fires at other venues shortly before the Whiskey Au Go Go attack that will provide an "important context".

The inquest was reopened after the firebombing was mentioned in a trial in which Vincent O'Dempsey and Garry Dubois were convicted over the deaths in 1974 of Barbara McCulkin and her two daughters.

That trial was told the killings may have been motivated over fears Ms McCulkin would try to implicate O'Dempsey in the firebombing.

Mr Keim said the evidence suggests O'Dempsey - who is expected to be a key witness - "was involved in organising other people to carry out the attack" rather than being present or active in starting the fire.

There is a "considerable amount of evidence" to support allegations that the attack was carried out by Finch, with Ms McCulkin's husband Billy and Thomas Hamilton, counsel assisting Avelina Tarrago told the inquest.

"Your Honour is likely to have evidence to consider indicating that the motive for the attack was to claim insurance and that a considerable number of people, other than Stuart and Finch, were responsible for the fire in that they contributed to arranging or planning it," she added.

"At the very least people knew the fire was going to happen."

She told the inquest it seemed there was "little effort to find other people involved" after the two men were arrested

More than 60 patrons and staff tried frantically to escape after two drums of fuel were thrown into the downstairs foyer of the nightclub in Brisbane's Fortitude Valley and set alight about 2am on March 8.

Fifteen people didn't make it out, dying from carbon monoxide poisoning.

Susan Hartley told of the inquest of having to identify her brother's body when she was just 17 years old.

"I then came home and had to tell my mother that her one and only son was dead," she said.

Denise Koch said the hurt of losing her sister Wendy Drew was still raw in her family, but she hoped people will be able to start to heal when the findings are released.

"There has been no resolution regarding responsibility for this awful tragedy," she said.

Mr Ryan will also look into the adequacy of investigations into the deadly attack immediately after the firebombing and over subsequent years.

The inquest was told former police officer Roger Rogerson, who is behind bars in NSW for murder and who was part of the initial investigation into the attack, may be called to testify.

Australian Associated Press