For a brief time, Juan Vallejo Corona was the most prolific mass murderer in American history. In 1971, police discovered the bodies of 25 migrant workers buried near Yuba City, California, who had contracted with Corona's business to work as fruit pickers in the area. Many had been hacked to death with a machete.

Corona was born in Mexico in 1934 and came to California in the early 1950s to work as a migrant laborer himself. He suffered from psychiatric problems and was diagnosed as schizophrenic. Corona nevertheless became a successful business owner by the late 1960s with a company that contracted with local orchards and groves to provide short-term harvest help. Many of Corona's hirelings were transients or Mexican immigrants, and Corona provided lodgings for them on his property.

In May of 1971, a nearby rancher noticed what appeared to be a freshly dug rectangular hole on his property. One of...