Home affairs correspondent, BBC News
It is said to be unusual for live ammunition to be used during training
A policeman has died in a shooting incident during a training exercise. Such shootings are extremely rare - the last time a police officer was fatally shot with a police firearm was in 1950.
Firearms training in England and Wales is one of the most closely regulated police specialisms.
The 6,700 officers authorised to use guns are required continually to refresh and update their skills.
As well as target-range practice, training consists of exercises to build up experience in hostage rescue and intercepting armed suspects and vehicles.
It is unusual for live ammunition to be used during such exercises; normally, blanks, plastic rounds or paint-firing bullets - "Simunition" - are used. All officers have to wear body armour.
But live ammunition is sometimes used to blast open doors if that is part of the training exercise.
Two training deaths
Accidents are believed to be comparatively rare - there are around two or three a year.
The most common scenario is an officer injuring themselves as they load or unload their weapon, or take it out of the holster.
Until the shooting in Manchester, only five officers had been accidentally shot dead with a police firearm in the history of the police service. Two of them died during training.
The last officer to die was Pc Samuel Lock, from the Metropolitan Police, in 1950, who was shot while cleaning a police pistol at a police station.
The last officer shot and killed by accident during a training exercise was Special Constable Arthur Guest in 1941, who was shot by an instructor.