A teenager has admitted to violently murdering a beloved Canberra watersports stalwart by stomping on the elderly man's head in a horrific attack, during which he made "prehistoric monster sounds".
The killer, who cannot be identified because he was underage at the time of the incident, pleaded guilty in the ACT Supreme Court on Wednesday to the murder of 82-year-old Richard John Cater.
He also pleaded guilty to the serious assaults of two other people in the same random attack, which prosecutors say was "the result of drug experimentation gone wrong".
Justice David Mossop has suppressed the identities of the assault victims, who were friends of Mr Cater aged in their 70s.
Agreed facts tendered to the court say that on March 15 last year, the three victims went out for dinner before heading for Mr Cater's home in the Gungahlin suburb of Palmerston.
As they were turning into the driveway, the teenager appeared, wrenched open the front passenger door of their car and began attacking Mr Cater's male friend.
He repeatedly yelled that he was going to kill this man as he bit his hand, tried to pull the sides of his mouth apart and punched him so hard he knocked out a tooth implant.
The car rolled forward into some bushes as Mr Cater's female friend, who had been driving, tried to push the teenager away.
The teenager then turned on the woman and began biting and kicking her.
At one stage, he grabbed her head and pulled it forward so hard he fractured her spine, causing long-term nerve damage.
Mr Cater, who was in the back seat of the car, got out and ran to grab a spade with which to defend his friends.
It is unclear exactly what happened in the seconds that followed, but the 82-year-old returned with the spade and ended up on the ground.
"[The male friend] saw Mr Cater laying in the gutter to the rear of the car and the offender stomp on his head; a blow that combined with others caused a severe head injury such that he never regained consciousness," the agreed facts say.
Mr Cater's male friend eventually managed to hit the teenager, who he described as making "prehistoric monster sounds", twice with the spade and the teenager fell to the ground.
"Whilst on the ground the offender was described as making noises like Jurassic Park," the agreed facts say.
When police arrived at the scene after being called by neighbours, the teenager lunged towards an officer and a violent struggle ensued, but pepper spray appeared to have no effect on him.
For the next hour and 40 minutes, four police officers fought with the teenager before they were finally able to handcuff and control him enough that paramedics could sedate him.
When he woke up in hospital, the teenager claimed to have no memory of what had happened.
Police executed a search warrant at the teenager's home two days after the attack and found three tabs of LSD, 6.9 grams of cannabis and a well-used bong made from a Gatorade bottle.
Investigators later learnt that around the time of the incident, the killer had been buying drugs from overseas on the dark net.
Hours before the attack, he took LSD at the Gungahlin McDonald's before smoking cannabis at his home and taking a second LSD tab.
"I'm tripping, this is crazy," he told a friend around that time, before vomiting as he "fell into a psychedelic crisis" before going to carry out the attack.
The teenager has been in custody ever since the incident and now faces a potential sentence of life in jail.
Justice Mossop ordered that the case go before a registrar on Thursday morning to set a date for sentencing.
He indicated that a full day would likely be needed to hear victim impact statements and submissions from legal counsel before a sentence could be handed down.