The incident in San Francisco on May 21 , 1971 was a murder .
Ex-militants charged in S.F . police officer 's '71 slaying at station , San Francisco Chronicle ( January 23 , 2007 ) On May 21 , 1971 , as many as five men participated in the murder of two New York City police officers , Joseph Piagentini and Waverly Jones .
Black Liberation Army