Exclusive: Kai Shannon has always maintained his innocence. There is now growing pressure to re-examine evidence with a former police officer on his side

When 18-year-old Wahab Zaaki was found bleeding to death from five stab wounds on an east London estate, police were under immense pressure to find his killer.

It was spring 2009 and knife crime in the capital had been branded an epidemic. Zaaki was one of four teenagers killed in separate incidents over just three days in London that March.