Just hours after police intercepted a device which they believe was about to be planted by dissident republicans in Londonderry, police revealed the discovery of a letter bomb at a postbox in a loyalist area of Co Down.
Detectives are questioning four men about the discovery of the bomb in Derry's Creggan estate on Thursday night. They were arrested when a car was stopped.
Security chiefs believe dissident republicans opposed to the peace process were planning an attack in the centre of the city, next year's UK City of Culture.
The discovery of the letter bomb at Clough, a loyalist area near Newcastle, Co Down, was disclosed by police two hours before Mrs Clinton's arrival. It was also found on Thursday night when officers were alerted to suspicious activity near the postbox.
Mrs Clinton's husband, former US president Bill, was a key player in the peace process during the 1990s. She has been to Northern Ireland several times before and will have been well advised before her arrival in Belfast about increasing loyalist anger on the streets because of the decision by the city council to limit the flying of the Union flag at City Hall. The discovery of another dissident republican bomb in Derry will also have caused alarm.
He said: "People are genuinely fed-up with this. It was freezing cold last night and there were people in their 80s and 90s who were moved from their home. Some forgot medication. The people they tell us they represent are the people who are being disrupted. This has to stop.
"Thank God the police found the bomb and the people of this city or somewhere else are not waking up to destruction today."