The Independent Police Complaints Commission is to use its own staff to probe how a police support worker was shot in a firearms training session.
The unnamed civilian employee was injured during a Thames Valley Police "firearms awareness training session" at Kidlington in Oxfordshire.
The wounded man remained in a serious but stable condition in hospital after the incident on Wednesday afternoon.
The IPCC was called in to investigate by Thames Valley Police.
'Criminal or disciplinary offences'
IPCC commissioner Deborah Glass said: "I have decided that we should conduct an independent investigation, using our own investigators, to establish the circumstances of this incident.
"The investigation will examine how live ammunition came to be present in a firearm during an awareness session, and consider whether any criminal or disciplinary offences have been committed."
The awareness session is part of an induction for police support staff to provide them with basic firearms awareness.
A Thames Valley Police spokesman said: "The session, held yesterday, was for call takers who have a key role to play to gaining information to assist in the force's spontaneous response to firearms incidents."
The sessions - run by fully trained firearms officers - are held throughout the year usually on a six to eight week basis.