Police have asked the public for help after a vulnerable man with the mental age of a nine-year-old child was found stabbed to death by a gang of "vicious cowards."
Dean Mayley, 24, was found collapsed with a suspected single stab wound to the chest after being attacked for no reason by three or four men in west London, police have said.
Officers said the young man would have had no chance of defending himself against the "sickening" gang who set upon him unprovoked in Greenford at 5.20pm on Friday evening.
Mr Mayley suffered from learning difficulties related to microcephaly which his family say left him very easily led and unaware of "how nasty the world is".
The last sighting of him before the attack was when he waved goodbye to the bus driver after missing his usual stop and getting off on The Broadway instead.
Mr Mayley's mother, Donna, pleaded for help in an emotional appeal in the hope of finding her son's killers.
"I hope there is somebody, somewhere, who knows something. I'm begging you, really - I don't know what to do," Mrs Mayley, 51, said.
"Dean was very vulnerable and easily led, he just wanted to please people all the time. He wanted to be an adult - to be like all the other lads.
"It took me a long time to be able give him that independence he wanted, bit by bit.
"He was vulnerable and didn't know how nasty the world is. Obviously now we know. What has happened to him is evil - but we need to know what happened."
Detective Chief Inspector Matt Bonner, of the Metropolitan Police’s Homicide and Major Crime Command condemned the attack, branding the men "vicious cowards".
"This was a vulnerable man who would have shown an unusual degree of trust in those who he came across and it is utterly sickening that he should have been set upon in this way.
"Those responsible are clearly dangerous and callous and it is critical that we track them down as soon as possible so that they can be held responsible for their appalling actions.
"These people are not worthy of protection and I would urge anyone with information to call police as soon as possible."
Detective Chief Inspector Bonner said Mr Mayley's family are distraught by his murder and urged anyone with information to speak to police.
"Think about the devastation this apparently random act of violence has caused," he added.
The incident room can be contacted on 0208 358 0200 or call Crimestoppers anonymously on 0800 555 111.