Three men have been arrested on suspicion of theft at the former MOD Bureau West site in Devizes.
The Wiltshire police helicopter was returning from a medical evacuation at around 1.30pm when the police observer noticed activity on the site, which has lain derelict for many years.
They were thought to be loading scrap metal from the buildings into an Isuzu Trooper 4x4. Officers from the neighbourhood policing team were soon on the scene and carried out the arrests.
The three men are in their 30 and early 40s. Two of them are from the local area and one from Sunderland.
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