ROB Knox's killer was being investigated by police at the time of the murder after an attempted robbery involving a knife.

But police did not arrest Karl Bishop over the incident, which happened in Eltham just two months before the stabbings in May last year.

After Mr Knox was killed, it was decided there was not enough evidence to proceed any further in the case of the attempted robbery.

Also in March last year, Bishop was a suspect in connection with a burglary in Chislehurst, but again police decided there was insufficient evidence to proceed against him.

An investigation by the Independent Police Complaints Commission into why Bishop was not arrested over the attempted robbery has led to two officers being given written warnings.

A police spokesman said: "Lessons have been learned from what happened in this case and measures have been taken, including the introduction of a new system to monitor centrally the progress of action to arrest suspects for all violent crime offences, including knife crime."