Two men have been arrested by detectives investigating the killing of a policeman in Northern Ireland. The province is reeling from the shooting, the second deadly attack on security forces in recent days. The officer was killed in the town of Craigavon. On Saturday two British soldiers were shot dead in Antrim. Two different Republican splinter groups have claimed responsibility.Amid concern the province could slide back into sectarian violence, political leaders in Belfast are calling for unity. Northern Ireland First Minister Peter Robinson said: “We all – as I had indicated yesterday – are being tested, and the stronger we stand together the more surely we will come through this together. We together call for anybody in our community who has the least bit of evidence to bring that evidence forward.” Investigations in the province are often hampered by a continuing climate of mistrust, and the British Prime Minister echoed calls for anyone with clues to contact the police. Gordon Brown said: “These are murderers who are trying to distort and disrupt and destroy a political process that is working for the people of Northern Ireland and I know the response of the people of Northern Ireland: they want peace and the political process to move forward; they do not want a return to guns on the streets.” The Real IRA, which was behind the killing of the two British soldiers, was also responsible for the province’s worst single atrocity: the 1998 Omagh bombing that left 29 people dead.