Mark Nowak (left) with son Henry. He said the 'evolution of what actually happened that night really shocked us, and we felt we were being hoodwinked.'Photograph: Family handout/PA

The family of the murdered teenager Henry Nowak, who was arrested as he pleaded for medical help after being stabbed, have said they were failed by the authorities and fear they were manipulated in their darkest hour.

His parents described how police wrongly told them Nowak, 18, had been the aggressor, and that officers had immediately administered first aid. They said the police watchdog had initially said there was no evidence of misconduct by officers.