Question: The San Quentin Six were six inmates at San Quentin State Prison in the U.S. state of California, they were accused of participating in an escape attempt, on which date, that left six people dead, including George Jackson, a co-founder of the Black Guerrilla Family?

full_context: Document 1: George Jackson (activist): George Lester Jackson (September 23, 1941 – August 21, 1971) was an African-American activist and author. While serving a sentence for armed robbery in 1961, Jackson became involved in revolutionary activity and co-founded the Maoist-Marxist Black Guerrilla Family. In 1970, he was charged, along with two other Soledad Brothers, with the murder of prison guard John Vincent Mills in the aftermath of a prison fight. The same year, he published "Soledad Brother: The Prison Letters of George Jackson", a combination of autobiography and manifesto addressed to a black American audience. The book would become a best-seller and earn Jackson personal fame. In 1971, Jackson took several guards and two inmates hostage in a bid to escape from San Quentin Prison. However, the incident ended with Jackson being shot and killed by a guard, in addition to the deaths of 5 hostages.
Document 2: San Quentin State Prison: San Quentin State Prison (SQ) is a California Department of Corrections and Rehabilitation state prison for men, located north of San Francisco in the unincorporated town of San Quentin in Marin County.
Document 3: Stephen Bingham: Stephen Mitchell Bingham (born April 23, 1942) is an American legal services and civil rights attorney who was tried and acquitted in 1986 for his alleged role in Black Panther George Jackson's attempted escape fifteen years earlier from San Quentin State Prison in Marin County, California, in 1971.
Document 4: Kevin Cooper (prisoner): Kevin Cooper (born 1958) is a death row inmate currently held in California's San Quentin Prison. Cooper was convicted of four murders that occurred in the Chino Hills area of California in 1983. Since his arrest, Cooper, who is African American, has become active in writing letters from prison asserting his innocence, protesting racism in the American criminal justice system, and opposing the death penalty.
Document 5: Brink's robbery (1981): The Brink's robbery of 1981 was an armed robbery and three related murders committed on October 20, 1981, which were carried out by six Black Liberation Army members: Jeral Wayne Williams (Mutulu Shakur), Donald Weems (Kuwasi Balagoon), Samuel Brown (Solomon Bouines), Samuel Smith, Edward Joseph, and Cecilio "Chui" Ferguson; and four former members of the Weather Underground, now belonging to the May 19th Communist Organization, consisting of David Gilbert, Judith Alice Clark, Kathy Boudin, and Marilyn Buck.
Document 6: Black Panther Party: The Black Panther Party or the BPP (originally the Black Panther Party for Self-Defense) was a revolutionary black nationalist and socialist organization founded by Bobby Seale and Huey Newton in October 1966. The party was active in the United States from 1966 until 1982, with international chapters operating in the United Kingdom in the early 1970s, and in Algeria from 1969 until 1972.
Document 7: Kuwasi Balagoon: Kuwasi Balagoon (December 22, 1946 – December 13, 1986), born Donald Weems, was a Black Panther, a member of the Black Liberation Army, a New Afrikan anarchist, and a defendant in the Panther 21 case in the late 1960s. Captured and convicted of various crimes, he spent most of the 1970s in prison. Balagoon escaped from prison several times, going underground and resuming BLA activity. He was finally captured and charged with participating in an armored truck armed robbery, known as the Brinks robbery (1981), in West Nyack, New York, on October 20, 1981, an action in which two police officers, Waverly Brown and Edward O'Grady, and a money courier (Peter Paige) were killed. Convicted of murder and other charges and sentenced to life imprisonment, he died in prison of pneumocystis pneumonia, an AIDS-related illness, on December 13, 1986, aged 39. Balagoon was bisexual.
Document 8: Soledad Brothers: The Soledad Brothers were three African-American inmates charged with the murder of a white prison guard, John Vincent Mills, at California's Soledad Prison on January 16, 1970. George Jackson, Fleeta Drumgo, and John Clutchette were said to have murdered Mills in retaliation for the shooting deaths of three black prisoners during a prison fight in the exercise yard three days prior by another guard, Opie G. Miller.
Document 9: Manson Family: The Manson Family was a quasi-commune that arose in California in the late 1960s, led by Charles Manson. They gained national notoriety after the infamous murder of actress Sharon Tate and four others on August 9, 1969 by Tex Watson and three other members of the Family, acting under the instructions of Charles Manson. Group members were also responsible for a number of other murders and assaults, and the attempted assassination of President Gerald Ford.
Document 10: George Jackson (song): "George Jackson" is a song by Bob Dylan, written in 1971, in tribute to the Black Panther leader, George Jackson, who had been shot and killed by guards at San Quentin Prison on August 21, 1971, during an attempted escape from prison. The event indirectly provoked the Attica Prison riot.
Document 11: George Jackson Brigade: The George Jackson Brigade, was a revolutionary group based in Seattle, Washington, which was named after George Jackson, a dissident prisoner and Black Panther member shot and killed during an alleged escape attempt at San Quentin Prison in 1971.
Document 12: Black Guerrilla Family: The Black Guerilla Family, or BGF (also known as the Black Family or the Black Vanguard) is an African-American prison and street gang founded in 1966 by George Jackson and W.L. Nolen while they were incarcerated at San Quentin State Prison in Marin County, California.
Document 13: Attica Prison riot: The Attica Prison riot, also known as the Attica Prison rebellion or Attica Prison uprising, occurred at the Attica Correctional Facility in Attica, New York, United States in 1971. Based upon prisoners' demands for better living conditions and political rights, the riot was one of the most well-known and significant uprisings of the Prisoners' Rights Movement. On September 9, 1971, two weeks after the killing of George Jackson at San Quentin State Prison, about 1,000 of the Attica prison's approximately 2,200 inmates rioted and took control of the prison, taking 42 staff hostage.
Document 14: Black Liberation Army: The Black Liberation Army (BLA) was an underground, black nationalist militant organization that operated in the United States from 1970 to 1981. Composed largely of former Black Panthers (BPP), the organization's program was one of armed struggle against the United States Government, and its stated goal was to "take up arms for the liberation and self-determination of black people in the United States." The BLA carried out a series of bombings, killings of police officers and drug dealers, robberies (which participants termed "expropriations"), and prison breaks.
Document 15: San Quentin Six: The San Quentin Six were six inmates at San Quentin State Prison in the U.S. state of California (Hugo Pinell, Willie Tate, Johnny Larry Spain, David Johnson, Fleeta Drumgo and Luis Talamantez), who were accused of participating in an August 21, 1971 escape attempt that left six people dead, including George Jackson, a co-founder of the Black Guerrilla Family.
ground_truth: August 21, 1971