U.S. 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 says.

Though state and federal officials have traced the current outbreak to a California company's fresh spinach, they have not pinpointed the source of the bacteria that have killed one person and sickened more than 100 others.

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

There is no evidence of tampering in the outbreak, FDA spokeswoman Susan Bro said Monday. That leaves a broad range of other possible sources, including contaminated irrigation water that has 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.

The FDA's 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.

"The FDA has not cleared any products from the list and continues to recommend consumers avoid eating fresh spinach products," she said.

Natural Selection has recalled 34 brands of fresh spinach products, distributed throughout the United States as well as to Canada and Mexico. The company said the manufacturing codes turned over to health officials from packages of spinach that had infected patients all were from non-organic spinach. The company packages both organic and conventionally grown spinach in separate areas at its San Juan Bautista, Calif. plant.

The brands include the company's own labels and those of other companies that had contracts with Natural Selection to produce or package its spinach.

Salinas-based River Ranch Fresh Foods recalled spring salad mixes containing spinach sold under the labels Hy-Vee, Fresh N' Easy and Farmers Market, FDA officials said. All contain spinach purchased from Natural Selection.

The FDA and California Department of Health Services were reviewing irrigation methods, harvest conditions and other practices at farms possibly involved.

Rep. Sam Farr, a Democrat whose district includes the Salinas Valley, said produce growers were keen to find the source of the contamination, given the crippling effects an outbreak can have on sales. He called prepackaged, fresh produce the latest innovation after canning and freezing.

"Obviously, it hasn't been perfected to get all the bugs out. But you don't have people fighting back out here. They're just saying, 'Help us, we want to get to the bottom of this,"' Farr said.

The spinach could have been contaminated in the field or during processing. About 74 percent of the fresh-market spinach grown in the U.S. comes from California, according to the California Farm Bureau Federation.

E. coli cases linked to the tainted spinach have been reported in 20 states, with Wisconsin reporting the most cases, including the death of a 77-year-old woman.

The Illinois Department of Public Health said Monday an elderly woman hospitalized with kidney failure is the state's first confirmed case in the spinach-linked outbreak. Other states reporting cases were 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.