Video of the incident where Met Police Sergeant Matiu Ratana shot by handcuffed suspect Louis De Zoysa on the chest at a custody centre while on duty died, has been released.
The gun used was an antique revolver that the suspect bought at an auction. Prior to arriving at the custody he had been searched but hid the gun under his armpit.
Mr De Zoysa, 23, had been arrested for multiple offences; possession of ammunition and possession of Class B drugs with intent to supply following a stop and search made by officers in the London Road area of Pollards Hill at 01:30 BST on Friday.
De Zoysa has pleaded not guilty to murder and is set to stand trial at Northampton crown court.