Staff at the Northern Ireland Mail Centre in Mallusk have intercepted a letter bomb sent to Chief Constable Matt Baggott.
The Mallusk sorting office was evacuated in the early hours of Friday morning (October 25) while Army Technical Officers dealt with the viable device.
The letter had been addressed to Mr Baggott at PSNI headquarters in Knock, east Belfast.
Secretary of State Theresa Villiers branded the incident “a callous and reckless attack”, while South Antrim MLA Paul Girvan described it as “despicable”.
His DUP colleague, Pam Brown MLA added: “I unreservedly condemn this cowardly act. Those who are responsible have nothing to offer our society as we strive to move forward together.
“These cowards do not care that innocent, hardworking people have had their lives put at risk and I hope the perpetrators will be brought to justice.”
Royal Mail confirmed that postal deliveries would be affected on Friday and apologised to customers for any inconvenience caused.
“Police would continue to ask members of the public to remain vigilant and anyone who notices anything suspicious should contact the police immediately,” a PSNI spokesperson said.