WASHINGTON — Members of extremist groups have signed up as school bus drivers in the United States, counterterror officials said Friday, in a cautionary bulletin to police. An FBI spokesman said "parents and children have nothing to fear." Asked about the alert notice, the FBI's Rich Kolko said "there are no threats, no plots and no history leading us to believe there is any reason for concern," although law enforcement agencies around the country were asked to watch out for kids' safety.