The main gate into an air base in west Suffolk had to be closed as a precaution after a vehicle tried to get in carrying a homemade firework.
Staff at the base invoked special control measures just before 8.30am this morning after finding the item during a routine inspection.
A spokesman from the base said the incident was not being treated as terrorist-related, or being linked to the discovery of devices at Ministry of Defence recruitment centres across the south over the past two days.
The main gate reopened at 11am.
The Ministry of Defence are now investigating the incident.
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