A gunman wearing an Afghan uniform has shot to death three American troops in the southern Helmand province, according to the U.S.-led international military coalition in the country, known as ISAF. U.S. officials told CBS News correspondent David Martin that the three slain troops were members of a Marine special operations force. Afghan sources in Helmand told CBS News Kabul bureau chief Fazul Rahim that they were part of a team overseeing the training and recruitment of Afghan local police. The sources said it was one of the police officers who opened fire on his mentors during dinner on Thursday evening. ISAF has not yet confirmed these details.