'Understaffing could see inmates take over a prison'
Four men were arrested after an explosive device was made at Cardiff prison.
South Wales Police said a bottle containing an unknown liquid "erupted and slid across the floor" on 15 June.
Three of the men are under investigation while one will face no further action. The incident only came to light when a prison worker contacted South West Wales Plaid Cymru AM Bethan Jenkins.
Ms Jenkins said under staffing played a part and this could lead to an incident where inmates take over a prison.