"The jury has arrived at a very quick verdict in this case and the reason is that these people were innocent of the charges put before the jury," the barrister Ken Archer said. 

"In my view the criminal justice system hasn't worked very well in this case."

Mr Archer said he planned take up his and the other lawyers' concerns over the charging, arrest and prosecution of the three Beowulfs with the ACT Attorney-General Gordon Ramsay.

"There were discretions exercised higher up that should not have been exercised, and they should never have been before the court," he said.

The portrait artist Melissa Beowulf, 61, and her sons Thorsten, 33, and Bjorn, 31, were accused of agreeing to murder Katherine Panin, 82, who was Mrs Beowulf's mother-in-law and the boys' grandmother. The three spent more than a year in jail before they were granted bail.

Mrs Panin had been reported dead from a fall down the back stairs of the family's Red Hill home. Her death was at first not suspicious, and it wasn't until later that police opened a murder investigation, prompted by what was alleged to be a lie about the time they had last seen Mrs Panin alive.

The Crown prosecuted a circumstantial case that sought to portray Mrs Beowulf as a woman driven by a desire to gain control over the family finances, who had become enraged after overhearing Mrs Panin make plans to change her will. Mrs Panin's estate was worth some $2.7 million. It was alleged there had been an altercation at the home on October 12, 2015, and Mrs Panin was hit to the head, by who the Crown could not say, before the accused staged the scene of a fall. The medical experts could not find a cause of death.

But over the course of the eight week trial the defence worked to dismantle the Crown's alleged motive, pointing out how the boys stood to gain from the change to the will by being made direct beneficiaries. Mrs Beowulf would see her stake only slightly diluted by the addition of two more beneficiaries for a total of seven - a small amount in the circumstances.

The defence were further assisted by the hours of phone tap evidence played to the jury. For months police had the family's phones, house and car bugged, unknown to the family, but the defence argued none ever acknowledged any role in any alleged murder or cover up.

At about 11.20am on Friday, after retiring late Thursday and spending about three-and-a-half-hours deliberating, the jury returned to court. One by one, the jury foreman delivered the verdicts of not guilty. This was, the judge had said, an all or nothing case. Either they were all guilty or none were. Mrs Beowulf, her verdict read first, nearly leapt out of her seat, while her sons stared ahead waiting to hear their own fates. 

Afterwards, they shook their lawyers' hands, while Mrs Beowulf wept and gripped her sons with hugs. They were joined by other family and supporters.

Outside court after the verdict, Mr Archer said the case should never have been brought.

"It's our intention to take it up with the attorney general in relation to why these people were charged, why they were kept in custody, and why the DPP persisted with a charge that clearly was never going to be proved," he told the media.

Mr Archer said he was not criticising the particular prosecutors involved, who he said had done their best. "But there were discretions exercised higher up that should not have been exercised, and they should never have been before the court."