David Eastman, a man who spent 19 years behind bars for the assassination of the Australian Capital Territory’s police chief, has been cleared of the crime in a dramatic retrial.

The verdict marks the end of another chapter in a case that has consumed Canberra since the night of 10 January 1989, when Colin Winchester was shot to death as he got out of his car outside his Deakin home.

Winchester, an Australian federal police assistant commissioner, remains the highest-ranking police officer to have been murdered in Australia’s history. The AFP said at the time that his murder marked “the end of the age of innocence for Australia”.

The case has seen it all. From evidence suggesting a Calabrian mafia revenge plot, to bogus forensic evidence, wrongful imprisonment and allegations of police misconduct in their almost three-year investigation.

At the centre of it all was Eastman, an economist and public servant who was charged with Winchester’s murder in 1993.

Eastman quickly came under suspicion after Winchester’s killing. The former Treasury official was facing an assault charge over a dispute with his neighbour.

The economist, fearing that the charge would ruin his career, tried desperately to have police drop it, and made representations to Winchester which were rebuffed. This, it was alleged, was Eastman’s motive. Evidence allegedly suggested that Eastman had made threats on Winchester’s life, had scoped out his home, and bought a gun that police believed to be the murder weapon.

Eastman was tried and convicted in 1995, and spent 19 years behind bars. He spent those decades protesting his innocence and fighting the charge in every conceivable forum, leading to a string of appeals and inquiries.

In 2014 an inquiry into his conviction declared that he had probably been responsible but had not received a fair trial. The inquiry completely debunked a crucial piece of forensic evidence used by police to link gunshot residue found in Eastman’s car boot to the murder scene.

The evidence was presented to the then jury as near-irrefutable proof that Eastman had been at the scene. Even the then trial judge, Ken Carruthers, paid tribute to the strength of the forensics when sentencing Eastman to life in November 1995.

“This investigation must surely rank as one of the most skilled, sophisticated and determined forensic investigations in the history of criminal investigation in Australia,” he said.

Eastman was freed in 2014 but placed on trial again, with prosecutors arguing there was enough residual evidence, even without the forensics, for a jury to find him guilty. The retrial was strongly supported by Winchester’s remaining family.

It is another day of mourning for the AFP and the Winchesters John Hinchey

The retrial in the ACT supreme court has spent months hearing from more than 100 witnesses this year, and a jury began deliberations last week, initially appearing to struggle to reach a verdict.

On Thursday morning they found Eastman not guilty.

Eastman said “thank you” to the judge after the verdict was read out.

The retrial had heard new evidence, largely in secret, about the possible involvement of the ’Ndrangheta – or Calabrian mafia – in Winchester’s killing.

Winchester had headed an undercover operation that brought down the mafia’s cannabis crop operations near Bungendore, in New South Wales, in the 1980s. The mafia had believed Winchester was a corrupt cop who had granted them protection.

Police have always maintained that they investigated the mafia links thoroughly at the time but could find no evidence of their involvement.

The Winchester family issued a statement saying they were “extremely disappointed”.

The ACT’s former victims of crime commissioner, John Hinchey, said police would also be heartbroken by the verdict.

“They would be heartbroken, I would believe, and grief-stricken, again,” he told reporters outside court. “It is another day of mourning for the AFP and the Winchesters.”

The AFP and ACT policing issued a statement saying their thoughts were with the Winchester family. The case had “placed strain on the Winchester family over a long period”, police said. The family will continue to receive support from the AFP.

“Chief police officer for the ACT, assistant commissioner Ray Johnson recognises the dedication and significant efforts of all those officers who have investigated this matter over almost 30 years,” the statement said.

“The AFP continues to have full confidence in the capabilities and professionalism of our investigators both now and in those who initially brought this case before the court.”

“In light of today’s outcome, the AFP will consult with its partners on any future actions.”