Mooney — A mural by Anton Refregier depicting San Francisco's 1916 bombing and the two men wrongfully accused of the act. The mural is on public display at San Francisco's Rincon Center.

Although incredibly infrequent, bombings in crowded public places are unfortunately not a new phenomenon in America. This week's Boston Marathon explosion harkens back to an often forgotten local tragedy nearly 100 years ago, when a bomb tore through downtown San Francisco during a major public event, killing 10 people and leaving scores of others seriously wounded.

The Preparedness Day Bombing, as it became known, was the worst act of terrorism in San Francisco's history. It occurred just after 2 p.m on July 22, 1916 during a huge San Francisco parade that had been organized to drum up public support for the United States' imminent entry into World War I. Not long after the 50,000 person march began, a huge blast echoed through the streets, set off by a pipe bomb filled with explosives and steel slugs that was hidden inside a suitcase and placed near the intersection of Steuart and Market streets, a stone's throw from the Ferry Building.

The following film, produced by the Hearst-Pathe News Service and shown to local audiences shortly after the tragedy, opens with a set of propagandist animation triumphing American prosperity and decrying the lawlessness and chaos that, it suggests, inevitably stem from radicalism. The film goes on to show actual footage of the parade and the chaotic scene in the explosion's aftermath.

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Like the Boston bombing, authorities had few concrete leads. Investigation was initially focused on local extremist political groups, who in the wake of labor unrest and the rise of Bolshevism, had spoken out vociferously against U.S. involvement in the war, and who the city's conservative business leaders eyed with growing concern.

With scant evidence, police arrested Thomas Mooney and his assistant Warren K. Billings, two well known radical labor leaders who had previously both been been arrested on attempted terrorism and civil disobedience charges (Mooney's wife was also arrested but later acquitted). The trial was hastily carried out in a lynch-mob atmosphere, in which the suspects were denied counsel. Both men were quickly convicted, with Mooney sentenced to death and Billings to life in prison.

In 1918 a commission reexamining the case, found no clear evidence of Mooney's involvement in the incident and commuted his sentence to life imprisonment. But over the next two decades, overwhelming evidence of perjury and false testimony during the trial prompted California Governor Culbert Olson to issue a pardon to both men.

Although theories among historians abound as to who the actual perpetrators were, the identity of the bomber has never been determined and will likely remain a mystery.

Anton Refregier, a New Deal artist, captured the scene of the bombing and subsequent trial (the composition at top). It's one of 27 murals depicting landmark events in California's history that the artist was commissioned to paint on the walls of a downtown San Francisco post office (now the Rincon Center) in the early 1940s. All of the murals remain on public view today. For a detailed audio tour of the works, including historical context, download KQED's Let's Get Lost app.

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The Bomb That Shook San Francisco A Century Ago