David Ford: Letter bombs are 'extremely worrying development'
29 October 2013 Last updated at 16:22 GMT
David Ford has said the increase in letter bombs being sent is an "extremely worrying development".
The justice minister was speaking as a fourth device was discovered in the post room of Stormont Castle, addressed to Northern Ireland Secretary of State Theresa Villiers.
He was speaking to BBC Newsline reporter Chris Page at a Royal Mail sorting depot in Mallusk, County Antrim.