A police officer has been shot dead in Northern Ireland, the third killing in as many days, raising fears of a return to sectarian violence in the province.The officer was killed while out on night patrol with colleagues in a predominantly Catholic area of Craigavon, a town 40 kilometres from Belfast. The group were fired on from nearby derelict ground. There has been no immediate claim of responsibility, but a senior politician said dissident Republicans were most probably behind the attack. Political reaction to the killings has been measured, and on a visit yesterday to the site of Saturday’s shootings in an Antrim barracks, British Prime Minister Gordon Brown said: “This attack has not happened because of the failure of the political process; it is in many ways because of the success of the political process that people are working together and a small number of people want to disrupt something that is working.” On Saturday, two British soldiers were killed at the entrance to their barracks by gunmen from the Real IRA, which also carried out the 1998 Omagh bombing. Security experts have questioned how capable the group is of launching a campaign of violence, given its small size. However, there are fears the killings could spark a backlash from the Protestant community.