Just 10 months before fresh spinach was implicated in a nationwide wave of E. coli infections, state and federal officials warned growers and packers in the Salinas Valley area to improve product safety, the San Francisco Chronicle said this week.

In November 2005, after a decade of outbreaks of the potentially deadly infection, the Food and Drug Administration wrote to the firms “to consider modifying their operations” to make sure their products were safe.

Investigators have traced the outbreak — the 20th in a decade — to Earthbound Farm’s Natural Selection label, grown in the Salinas Valley area. Natural Selection has voluntarily recalled all its spinach products. State officials urge the public not to eat raw spinach of any kind, though canned and frozen spinach are safe.

“We are trying to get to the bottom of this,” said state public health officer Dr. Mark Horton. “But we have not been able to identify a smoking gun. A lot more has to be done.”

Meanwhile, some farm workers are being laid off or given other assignments, the United Farm Workers union said. As long as the harvesting season continues, workers said, they can shift to other jobs, but things will be more difficult after the harvest season ends next month.