The Earthbound Farm/Natural Selection Foods plant is seen in San Juan Bautista, Calif., on Friday. The natural foods company was linked Friday to a nationwide E. coli outbreak that has killed at least one person and sickened more than 100.

Washington ? 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.

Though state and federal officials have traced the current outbreak to a California company’s fresh spinach, they haven’t pinpointed the source of the bacteria that have killed one person and sickened at least 113 others. A second death was being investigated in the outbreak, which involves 21 states.

The FDA is still warning consumers not to eat fresh spinach.

The regulatory agency does not consider the contamination deliberate.

That leaves a broad range of other possible sources, including contaminated irrigation water that’s been a problem in California’s Salinas Valley. The area on California’s central coast produces much of the U.S. spinach crop.

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.

“In light of continuing outbreaks, it is clear that more needs to be done,” the FDA’s Robert Brackett wrote in a Nov. 4, 2005, letter.

Suggested actions included discarding any produce that comes into contact with floodwaters. Rivers and creeks in the Salinas watershed are known to be periodically contaminated with E. coli, Brackett said.

Meanwhile, the FBI is monitoring the situation, said spokesman Rich Kolko. He called it a routine and precautionary measure, not an indication of suspicious activity.

FDA spokeswoman Susan Bro dismissed a claim by Natural Selection Foods LLC, the company linked to the outbreak, that its organic spinach products had been cleared of suspicion.