COLUSA COUNTY (CBS13) – A man has been convicted of murdering a farmworker by setting a bomb in an electrical panel that was triggered by the victim at irrigation pump.

Paul Moore of Grimes, Calif. was found guilty on Friday of setting the explosive device that killed Roberto Ayala, 43, on July 16, 2011.

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Ayala was killed after he turned on an irrigation pump at a rice field at the Moore Brothers farm in Colusa County. Information later obtained from PG&E indicated the explosion wasn’t merely a tragic accident.

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According to the Bureau of Alcohol Tobacco and Firearms, the Colusa County Sheriff’s Office received a letter on August 12, 2011 claiming responsibility for building and detonating the bomb. On August 15, 2011 they received another letter that contained a diagram of the device. On December 6 of that same year, agents searched Moore’s home and found evidence linking him to the crime, they say.

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Ayala’s 7-year-old son was with him when the explosion occurred and he ran about 2 miles through farmland to seek help for his father, who died at the scene.