A solemn memorial service was held Sunday for the four UN peacekeepers who were killed last week when an Israeli shell hit their post in southern Lebanon. About 70 people, including UN officials and relatives of one of the four killed peacekeepers, attended the service in Jerusalem. The four peacekeepers were from Austria, Canada, China and Finland. Prime Minister Ehud Olmert has expressed "deep regret" for the deaths. He told Annan that the attack was inadvertent and that he promised a "thorough investigation," his office said in a statement. However, in a letter read at Sunday's memorial service, Annan said: "Many questions surround the tragic deaths of these unarmed peacekeepers, and these must be answered."