A fifth soldier has been arrested on suspicion of manslaughter following the death of a 22-year-old serviceman.
There are 600 Royal Welsh soldiers based at the barracks
The 29-year-old and four colleagues aged 44, 35, 31 and 28, are being questioned by Wiltshire Police.
The men were arrested after the young serviceman died at about noon on Monday, following unofficial exercises on Salisbury Plain.
All were with the 2nd Battalion Royal Welsh Regiment based at the Lucknow Barracks in Tidworth.
'Hot and agitated'
A Ministry of Defence spokesman would not be drawn on reports that the soldier had suffered heat exhaustion.
Wiltshire Ambulance Service said they were called to the Queen Elizabeth Medical Centre at the barracks where they found him unconscious.
It is understood he came around briefly and was delivered to Salisbury hospital at 1340 BST, but subsequently died.
The soldier was said to have been "hot and agitated" when he was taken to hospital.
A post-mortem examination is due to be carried out by a Home Office pathologist.
The Ministry of Defence said it was taking the incident "extremely seriously", but added that it was for the police to comment on the investigation.
The four men arrested following the death were left to sleep overnight before police began questioning them on Tuesday.
The 2nd Battalion The Royal Welsh recruits was founded 300 years ago and has 600 members based at Lucknow Barracks in Tidworth.
A Welsh flag was being flown at half-mast at the entrance to the barracks on Tuesday.