Letter bombs have been sent to DUP deputy leader Mr Peter Robinson and to Policing Board member Mr Alex Attwood of the SDLP.
The bomb addressed to Mr Robinson was handled at his Belfast constituency office, while that addressed to Mr Attwood was delivered to the Policing Board headquarters in Belfast's docklands yesterday.
The devices were said to have been crudely constructed and to have contained explosives and ball bearings. No-one was hurt.
Mr Robinson denounced those responsible: "It's particularly despicable because anybody who sends that kind of device would know that it would not be opened by the member of parliament himself but by a member of his staff."
"For someone to attempt to kill a member of staff because they disagree with me indicates the kind of mind we are dealing with."
Ms Rebecca Robinson, who works for her parents, MPs Peter and Iris, discovered the device.
British army technical specialists were called in to make the device safe.
Mr Robinson said he suspected dissident republicans were responsible.
Mr Attwood declined to comment last night.