Federal health officials told California farmers to improve produce safety in a pointed warning letter last November, nearly a year before the multistate E. coli outbreak linked to spinach.

In fact, the current food-poisoning episode is the 20th since 1995 linked to spinach or lettuce, the Food and Drug Administration said.

There have been 19 other food-poisoning outbreaks since 1995 linked to lettuce and spinach, according to the FDA. At least eight were traced to produce grown in the Salinas Valley. The outbreaks involved more than 400 cases of sickness and two deaths.

In 2004 and again in 2005, the FDA’s top food safety official warned California farmers they needed to do more to increase the safety of the fresh leafy greens they grow.

On Monday, Illinois and Nebraska joined the list of states with confirmed cases. Those states are: California, Connecticut, Idaho, Indiana, Kentucky, Maine, Michigan, Minnesota, New Mexico, Nevada, New York, Ohio, Oregon, Pennsylvania, Utah, Virginia, Washington and Wyoming, according to the Centers for Disease Control and Prevention.