A human rights group has urged Kosovo authorities to investigate claims by a former U.N. war crimes prosecutor that ethnic Albanian guerrillas killed dozens of Serbs and sold their organs at the end of the war in Kosovo. New York-based Human Rights Watch said Carla Del Ponte has presented sufficiently grave evidence'' in her newly published book to warrant an investigation into claims guerrillas took Serbs into Albania, killed them and then sold their organs to international traffickers in 1999. In a letter dated April 4 and addressed to Kosovo's prime minister, the rights group called upon Kosovo's authorities to determine the veracity'' of the claims with counterparts in Albania.