Cassius Turvey, a 15-year-old Noongar Yamatji boy, died in hospital 10 days after prosecutors say he was chased, knocked to the ground and "deliberately struck to the head" in Perth's eastern suburbs on October 13, 2022.
Security guard Edward Fischer found the teen after the alleged attack in the car park of a TAFE college where he worked.
"Around his ears there was blood. The back of his head there was blood," he told the West Australian Supreme Court from the witness box on Tuesday.
"It looked like one ear ... had been hit with a cookie cutter.
"The other ear looked like it wasn't completely on."
Aleesha Louise Gilmore, 23, her boyfriend Jack Steven James Brearley, 23, and his mates Brodie Lee Palmer, 29, and Mitchell Colin Forth, 26, are on trial, charged with murdering Cassius.
Mr Fischer said he helped Cassius while one of the children with the teen called triple zero for an ambulance.
"The kids were pretty scared ... That someone was going to come and hurt them again," he said.
He told the court that while they were waiting for paramedics, a black four-wheel drive pulled into the car park.
"They thought it was the people that were originally chasing them," Mr Fischer said of the children.
"I could see people in it. I can't describe them but I could see ... there was three people in it.
"They all looked male."
Mr Fischer said the vehicle "revved its motor a few times" before driving off.
A short time later the ambulance arrived and paramedics took out the gurney but quickly got back in the vehicle and drove off without Cassius.
"She said something about a knife," Mr Fischer said, adding he hadn't seen a weapon.
The jury heard Cassius was, by this time, "in a lot of pain" and Mr Fischer called the ambulance service again.
"He was yelling out ... He did say it was hard to breathe and he couldn't think ... that his head was hurting a lot more," the security guard said.
The ambulance later returned with police.
Paramedic Erin Olearhead said some of the young people near Cassius were angry when she and another paramedic attended the first time.
"A bystander was holding a knife ... He was waving it around," she said.
"It was about 15cm long including the handle.
"Neither of us felt safe so we wanted to withdraw."
She said Cassius wasn't threatening, he was calm and compliant.
Prosecutors allege Brearley struck Cassius in the head twice with a metal shopping trolley pole while "hunting for kids" because somebody had smashed his car windows.
They allege Forth, Palmer and Gilmore helped him and knew his intent before the incident.
The trial continues.
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