A prisoner killed a female Serco guard by kicking her in the head like a "football", a jury has found.
Law degree drop-out Humphrey Burke, 23, carried out the fatal attack on 54-year-old grandmother Lorraine Barwell at Blackfriars Crown Court on June 29 last year.
Burke, formerly from Friars Wharf, Oxford, was ruled unfit to stand trial for Mrs Barwell's murder as he has since been diagnosed with paranoid schizophrenia.
Following a trial of the facts at the Old Bailey a jury deliberated for just 20 minutes to decide that he did commit the act.
The known troublemaker had started acting up as Mrs Barwell handcuffed herself to him to escort him to a prison van along with two other female officers, one of whom was "shadowing" on her first day in the job.
Male officers stepped in to help restrain Burke and get him to his feet now handcuffed behind his back after he appeared to collapse on the ground.
The court heard that Mrs Barwell then lost control of his head and he kicked her twice, once in the arm or body and then a devastating blow to the head, causing catastrophic brain injuries.
New starter Gia Sofokli was standing half a metre behind the victim.
She said: "He was really quite tall and she was really quite small.
"She reached out in front of her and up and that's when he kicked her for the first time. He just literally pulled his leg back and kicked her.
"She fell backwards about half a metre in front of me. She screamed quite loud and then it was about two seconds before he swung again with his foot but this time it got her in the face.
"She was on the floor in a foetal position. It looked like a footballer kicking a football really hard. The noise was quite loud. Her head went back but it did not smack the floor. It just snapped back. He was looking straight at her.
"She made a rumbling noise. That's when I looked down and saw blood. He split her lip when he kicked her.
"I immediately said 'shall I call an ambulance' but in that moment when I looked down I went into shock. My manager was speaking to me. I could hear her but I couldn't move."
Mrs Barwell died in hospital from a brain haemorrhage two days later.
Afterwards, the defendant said: "I did not intend to murder that woman. That's all I have to say."
Prosecutor Duncan Penny QC said Burke had been "unpredictable" at previous Blackfriars hearings where he had pretended to faint, collapse and go limp.
On the morning of the attack, his file had been marked "three-man unlock" meaning three officers had to be present when his cell was unlocked.
Mr Penny said "some of the staff" were aware of details of his previous behaviour.
He told the court that at the time of the attack, a psychiatric report had been prepared on Burke but no definitive diagnosis of mental illness had been made.
He had pleaded guilty to the attempted robbery of two bookmakers in Islington in 2014 as well as arson and criminal damage.
In one of the incidents, he had threatened staff with a claw hammer and damaged Perspex screens and in the other, he had set fire to a shop using white spirit.
Burke, who is currently at Broadmoor secure hospital, sat emotionless in the dock throughout the four-day trial of facts flanked by five guards.