Dozens of police are also combing a sprawling farm estate at Staplehurst in the county of Kent, in England's south-east, in the hunt for the gang.
Police were confident they were hot on the trail of the gang who raided a Securitas cash depot in Tonbridge, Kent.
The mob kidnapped the depot director, his wife and nine-year-old son at gunpoint on Tuesday in the dead of night.
Police believe the white Renault truck, a key piece in the jigsaw, was used to load the money at the depot. They also believed it was used to dump money crates in fields elsewhere in Kent.
At the Staplehurst farmhouse officers carried out a sweep across the grounds and police divers searched an ornamental well.
Kent police said they had been given permission by magistrates to keep questioning four people already held in custody.
Meanwhile police said another suspect, a woman, was arrested on Tuesday in relation the heist.
Raids were continuing across Kent and elsewhere in south-east
England as the police chase leads following a deluge of calls from the public.
Securitas risk manager Paul Fullicks said the 14 depot staff held in the raid had described their ordeal as “brutal, horrific and traumatic” were intimidated during the armed robbery.