Mafia drug growers "had a sufficiently strong motive" to assassinate police chief Colin Winchester, David Eastman's defence team says.
Defence counsel George Georgiou, SC, on Thursday said 11 men - with alleged Mafia links - caught in a police sting had all lost money and faced time behind bars on serious charges.
Mr Georgiou used the final hours of his more than week-long closing submissions to explore a possible link between Italian organised crime and Mr Winchester's 1989 murder.
Mr Eastman, 73, is on trial in the ACT Supreme Court after pleading not guilty to gunning down Mr Winchester after he pulled into his neighbour’s Deakin driveway about 9.15pm on January 10, 1989.
The prosecution alleges Mr Eastman murdered Mr Winchester in an act of revenge after developing a murderous hatred of police due to perceived injustices committed against him.
At the time, the former Treasury official was facing an assault charge and feared a conviction would ruin his bid to return to work in the public service.
The court has previously heard that, after the murder, police pursued multiple lines of inquiry and multiple potential suspects, including bikies, police, ex-police, and the Mafia.
Mr Winchester, an Australian Federal Police assistant commissioner, had previously worked on an operation observing cannabis growers in Bungendore - who believed they were under protection from authorities.
The cannabis growers were later arrested and put before court, and police understood there was a belief that Mr Winchester had double-crossed them.
The court heard investigators probed the possibility of Mafia involvement in the hit, including sending officers to Italy to speak with Italian organised crime experts.
Mr Georgiou said lead investigator Mr Ninness had been told the shooting had the hallmarks of a Mafia hit.
In August, Mr Ninness told the trial police had also bugged the homes and phones of Italian figures, but found no evidence to assist the murder investigation.
However, Mr Georgiou on Thursday said: "The fact they could not find a link does not mean there is not one there."
The silk said the Bungendore drug growers, who were due to face committal in the weeks after the murder, all had a strong motive.
"Perhaps one founded on revenge or payback, or sending a message to others" who would try and penetrate their "secretive society", Mr Georgiou said.
"They were all facing serious charges."
Acting Justice Murray Kellam began his final directions to jurors late on Thursday afternoon.
The judge's instructions are expected to last several days.
The trial continues.