DUBLIN — A letter bomb was sent to Britain’s top official in Northern Ireland on Tuesday, forcing an evacuation at the sprawling estate near Belfast that houses the offices of the province’s political leaders until the parcel was defused by the British Army. It was the fourth letter bomb mailed to a high-profile figure in Northern Ireland over the past week.

Dissidents hostile to the peace process between Britain and Northern Ireland who many believe to be former Irish Republican Army members are suspected. None of the letter bombs have detonated, but the rash of mailings appeared to be part of an expanded effort by the dissidents to disrupt the peace process, which they say entrenches British rule and diminishes their aspirations for a united Ireland.

The latest letter bomb was addressed to Theresa Villiers, the secretary of state for Northern Ireland, who is responsible for overseeing the peace process and representing Britain’s interests, including security. She was not at the estate, Stormont Castle, when the parcel was received by its postal sorting office.

“Those responsible for sending this, and other devices, through the post have absolutely no regard for the lives of postal workers and staff working in offices,” said Peter Robinson, the province’s first minister, who was among those evacuated from Stormont. “Northern Ireland will not be dragged back by terrorists who have nothing but misery to offer.”