During the jail sentence of White Panther John Sinclair a "Free John" concert took place, including John Lennon and Stevie Wonder. Sinclair was released two days later. On August 29, three BLA members murdered San Francisco police sergeant John Victor Young at his police station. Two days later, the "San Francisco Chronicle" received a letter signed by the BLA claiming responsibility for the attack. Late in the year Huey Newton visited China for meetings on Maoist theory and anti-imperialism. Black Power icon George Jackson attempted to escape from prison in August, killing seven hostages only to be killed himself. Jackson's death triggered the Attica Prison uprising which was later ended in a bloody siege. On November 3, Officer James R. Greene of the Atlanta Police Department was shot and killed in his patrol van at a gas station by Black Liberation Army members.