Police suspect the theft of Mary Queen of Scots rosary beads from Arundel Castle was carried out with inside knowledge.
Sources say the intruders appeared to know how to avoid security cameras, and even disabled some of those covering key areas during the £1million raid.
Detectives believe the criminals must have conspired with someone who has detailed information about the historic property.
The burglars may also have known that the Duke of Norfolk, 64, and his family would be away from the castle in West Sussex, which has a history dating back to 1067.
Two security guards were on duty on Friday night when the gang broke in through a window and forced open a display cabinet, seizing the gold beads and other royal treasures, including coronation cups spanning several centuries.
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