A bomb was found as sectarian tensions grew in advance of U.S. Secretary of State Hillary Clinton\'s visit to Northern Ireland, officials said. Soon after police discovered an explosive device they said likely was about to be planted by dissidents in Londonderry Thursday, police said they found a letter bomb in a mailbox in Clough, the Belfast Telegraph reported Friday. Clinton arrived in Dublin Thursday to attend a security conference. She was to meet with Northern Ireland First Minister Peter Robinson and Deputy First Minister Martin McGuinness at Stormont Castle in Belfast before returning to the United States. The discovery of the letter bomb at Clough was revealed by police two hours before Clinton\'s arrival. It was found Thursday when officers were alerted to suspicious activity near the mailbox. Security officials said they believe dissident republicans opposed to the peace process were involved. Clinton\'s husband, former U.S. President Bill Clinton, had a key role in the peace process during the 1990s.