The PSNI has intercepted two letter bombs addressed to Chief Constable Matt Baggott and another serving senior officer.
Police said the device sent to Mr Baggott was discovered at a Royal Mail sorting office on the outskirts of north Belfast.
Army bomb disposal experts were called to Mallusk, Co Antrim, during the early hours of this morning and worked through the night to make the device safe.
The package had been addressed to Chief Constable Baggott at the PSNI headquarters in Knock, on the edge of east Belfast.
The second letter bomb was sent to an unnamed serving senior officer and intercepted at a sorting office in Lisburn, Co Antrim.
Army bomb disposal experts defused the device this morning.
PSNI Assistant Chief Constable Will Kerr said: "It is a definite line of inquiry at this stage that it is a dissident republican grouping of some sort."
Mr Kerr said it was fortunate no-one was killed or seriously injured.
He insisted that morale was high among rank-and-file officers and said they would not be deterred from doing their duty.
"We will continue to deliver a policing service to the people of Northern Ireland irrespective of this threat," Mr Kerr said.
"We have been very successful and we will continue to be successful in disrupting the activities of these small groups."
Northern Ireland Secretary of State Theresa Villiers said those behind the letter bomb sent to Mr Baggott had no support.
She said: "The chief constable undertakes a challenging role in delivering community policing in a difficult threat environment.
"He and his officers have my full support and I am certain that the vast majority of people in Northern Ireland will condemn this disgraceful bomb attempt and others which have taken place over recent days."
These are the latest attempted bomb attacks in Northern Ireland in the past week.
Police officers escaped injury when pipe bombs were thrown at patrols in Newtownabbey, Co Antrim, and Derry while army technical experts made safe another viable pipe bomb in north Belfast overnight.