A dozen envelopes containing bullets and addressed to Sinn Féin representatives have been intercepted by Royal Mail workers.
The police were called to the sorting office at Mallusk, Co Antrim, last night after staff discovered the suspicious packages.
One of the envelopes was addressed to Sinn Féin Assembly member for North Antrim Mr Philip McGuigan.
Mr McGuigan said it was the latest in a number of threats made against him and called on DUP leader Dr Ian Paisley, the MP for North Antrim, to condemn the incident.
He said: "Over the past number of years, unionist paramilitaries have left a bomb at my home and issued numerous death threats against myself and my young family.
"In the past, when my own home has been attacked or that of party colleagues in Ballymena and elsewhere in North Antrim, Ian Paisley has remained silent.
"For nationalists and republicans living in North Antrim, this silence says much."