Guns and a quantity of military-grade explosives were discovered on Wednesday night near the Maximum Security Prison (MSP), Arouca.
Acting on a tip-off, members of the Special Operations Response Team (SORT) went to the prison’s garden area at around 11:40pm and found the items hidden amongst the crops.
An AR 15 assault rifle, two pistols, a revolver and a quantity of C4 plastic explosives with detonators were found and seized, along with two wireless radios.
It is suspected that the weapons were stashed there for the low-risk prisoners who are part of the gardening programme to smuggle inside after working there.
Investigations are ongoing.