A Prison Service source suggested the prison van ambush would probably involve one of the escapees having access to a smuggled mobile phone.
A phone call could then be made or a text sent to let the armed gang know which prison van they were in and so which vehicle to target, among the number of vans making their way from jail to Manchester Crown Court.
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Surrey police believe the deaths may be connected to the death of a 50-year-old man in southern France.
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