Most fresh spinach in the United States is "as safe as it was" before a nationwide E. coli outbreak, the U.S. Food and Drug Administration said, revising a two-week-old consumer warning.

The warning now covers only specific brands packaged on certain dates. Consumers should continue avoiding spinach recalled earlier this month by Natural Selection Foods LLC of San Juan Bautista and four companies that it supplied, said Kevin Reilly, deputy director of prevention services for the California health department.

A week ago, the FDA had said it was safe to eat spinach grown anywhere outside of three Salinas Valley counties, and some stores began restocking. Dr. David Acheson, chief medical officer of the agency's Center for Food Safety and Applied Nutrition, said Friday that only spinach that had already been recalled shouldn't be eaten.

Other spinach "is as safe as it was before this event," Acheson said.

Natural Selection's recall covered 34 brands it packaged bearing "Best if Used By" dates of Aug. 17 through Oct. 1. Reilly said the recall and the expiration dates make it likely the tainted spinach that led to the E. coli outbreak has mostly worked its way out of the food supply.

River Ranch Fresh Foods of Salinas, Pacific Coast Fruit Co. of Portland, Ore., S.T. Produce of Seattle and RLB Food Distributors of West Caldwell, N.J., recalled products that contained spinach supplied by Natural Selection.

Natural Selection and nine farms that supplied the company with spinach have been the focus of inspectors searching for the source of the outbreak, which has sickened 187 people and killed at least one in 26 states. Nine packages of Dole baby spinach, one of the brands Natural Selection bagged, have tested positive for E. coli.

The FDA action came on the same day California growers and produce processors presented preliminary food safety guidelines the agency set as a prerequisite for removing its spinach warning after the E. coli outbreak.

But state health officials said the FDA's decision to tell consumers they could resume eating all but a very limited supply of spinach was unrelated to the industry blueprint submitted earlier in the day.

"Our recommendation is that the public should not consume the spinach specifically cited in the recall," Reilly said. "As I've said pretty persistently, there are some concerns about systemic contamination in the Salinas Valley, and it's really critical the farms, the ranchers, implement good agricultural practices routinely and on every farm, every day."

The recent outbreak is the 20th connected to leafy green vegetables -- lettuce or spinach -- since 1995, half of them linked to farms in central California's Salinas Valley. Investigators don't yet know how the greens linked to the current outbreak became tainted, and may never know for sure, Acheson said.