A letter bomb intercepted by postal workers in Lisburn on Friday was addressed to Derry commander, Chief Inspector, John Burrows, the Sunday Journal understands.
A device, addressed to Chief Constable Matt Baggott was also intercepted in a sorting office in Mallusk on Friday.
PSNI Assistant Chief Constable Will Kerr said both devices were designed to kill.
“We are very lucky we are not dealing with a fatality,” he said. “It is a definite line of inquiry at this stage that it is a dissident republican grouping of some sort.”
A PSNI spokesperson said: “The second package was addressed to a senior police officer. Police are not in a position to confirm the name of the recipient.”