Police failed to seize several opportunities that could have prevented the murder of an Adelaide teenager, a coroner has found, admonishing them for a "lack of vigour, leadership and cohesion" in the case.
Lewis McPherson, 18, was shot by a drugged and drunk acquaintance, Liam Humbles, then 17, as he walked to a New Year's gathering with friends in 2012.
Deputy Coroner Anthony Schapel said on Friday that Humbles had the gun he used in the killing for at least six months, and police had been told about the firearm and his drug dealing.
The opportuntities included police speaking with Humbles' alleged associate, arresting Humbles when he was found with cannabis instead of issuing his fourth diversion order and searching his home address in August when they were told he was dealing drugs.
He said police could have "investigated that information with vigour" but did not, a trend that continued in a number of encounters with Humbles that year.
"The matter of Liam Humbles throughout 2012 was met by police with a lack of vigour, leadership and cohesion," Mr Schapel said.
"In the court's opinion there is no question that the firearm that Liam Humbles maintained in his possession during the course of 2012 could have and should have been located by police."
Responding to the coroner's criticism, SA police said they would thoroughly review the findings and recommendations before commenting further.
Humbles was jailed for at least 23 years, reduced to 17 years on appeal, for Mr McPherson's murder as well as the attempted murder of two other men.
His case helped drive changes to gun laws in South Australia, with mandatory prison terms introduced in 2014 for anyone who sold an illegal firearm, regardless of whether it was used in a crime.
In his findings, Mr Schapel outlined nine opportunities that could have prevented Mr McPherson's death, most of them in relation to the police.
"I am not certain that all of them if taken singularly would have prevented his death, but when examined collectively there seems to be little doubt about it," he said.
Mr Schapel made 17 recommendations to prevent a similar death, including that police give "appropriate priority" to investigating allegations of unlawful possession of firearms.
And he recommended the drug diversion process for children be "re-evaluated in its entirety" with a limit placed on the number of diversion notices that any child could receive.
© AAP 2017