The distraught family of a murdered man with the mental age of nine fled court in tears today as "chilling" footage capturing the moment he was stabbed to death was played at the Old Bailey.
There was a loud shriek of horror from the public gallery as jurors saw the moment Dean Mayley, 24, was surrounded by three "shadowy figures" and slashed with a knife by a hooded man.
The vulnerable young man then staggers into the street and collapses in front of oncoming traffic, nearly being hit by a car.
Mr Mayley, who was described as "vulnerable" due to a brain disorder, died in hospital a couple of hours later.
As the shocking footage was played Mr Mayley's mother and sister Emma ran out of the courtroom in tears, his mother clutching a tissue to her face.
Prosecutor Simon Denison QC told the court: "They are going after Dean Mayley, and knowing what is about to happen it is a chilling piece of footage to watch."
Mr Denison said it is believed the gang targeted Mr Mayley because they wanted to steal his mobile phone and North Face jacket as he walked home on February 7.
The clip shows three teenagers getting out of a blue-grey Fiat Punto in the same street Mr Mayley was walking home.
Shocking images from CCTV security camera on the front of a private house in Ruislip Road later allegedly shows one of the teens running up behind Mr Mayley with a knife clenched in his right hand.
Meanwhile two other men can be seen standing menacingly in the road blocking Mr Mayley's path, effectively cornering him.
The hooded figure, who cannot be named for legal reasons, then lashes out and plunges the blade into Mr Mayley's heart, it is alleged.
The trio run off while the 24-year-old manages to stumble into the road before collapsing in front of a car.
Mr Denison told the jury: "The image shows (the 17-year-old) confronting Dean Mayley. He has obviously said something to attract Dean's attention, Dean stops and turns.
"The (17-year-old) went up to him and in his right hand, clearly visible in his right hand behind his back, is a knife."
He said the two other teens stood in the road "to intimidate him and to provide encouragement and if necessary assistance" to the alleged killer as he pounced at around 5.20pm.
Mr Denison added: "And then suddenly, with Dean just stood there, (the 17-year-old) struck out at him with the knife in his right hand and stabbed him straight into the heart.
"There was no reaction (from the other two teens) to indicate any surprise at what had happened.
"All three immediately and together ran off back in the direction they came from."
Two women got out of their car and rushed over to try to help and comfort Mr Mayley, who was still conscious but bleeding on the floor.
They called an ambulance and emergency surgery was carried out at St Mary's Hospital in central London. But he died hours later from the 9cm stab wound.
Mr Denison QC had earlier said it may well be that Mr Mayley's mental condition meant that he did not understand what was happening, or what they wanted.
Four teenagers aged between 17 and 19 are on trial for the attempted robbery and murder of Mr Mayley.
The 17-year-old who allegedly struck the fatal wound had only celebrated his birthday a few days before, the court heard.
Another 17-year-old and Ryan Beresford, 19, stood in Mr Mayley's way, and 18-year-old Michael Mensah drove them all to and from the scene, it is alleged.
Beresford, of Acton, west London, and Mensah, of Greenford, west London, along with the two 17-year-old boys, who cannot be named, all deny murder.
Beresford, Mensah and one of the 17-year-olds deny attempted robbery. But the 17-year-old who allegedly stabbed Mr Mayley has admitted the charge of attempted robbery, the court heard.
Mensah alone is accused of an earlier robbery on January 28. He denies the charge.