The device fired at the security base in Bessbrook, Co Armagh, yesterday was a Mark 15 'barrack buster' mortar bomb containing 150 pounds of explosives packed into a gas cylinder, the RUC said late last night.
It was fired from a white van which had been abandoned near the base with a hole cut in the roof for the mortar to be fired through.
Despite it failing to explode it posed a "significant threat" to people in the area, said police.
RUC Insp Ralph Taggart said: "It was a significant device prepared by people intent on causing death and distruction."
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Residents evacuated from their homes were allowed to return last night after army bomb experts defused an unexploded device in the yard of a local commercial premises in the village.
PA