Although the investigation remains preliminary, officials suspect the virulent strain of E. coli bacteria came from pre-washed spinach, infecting people in Connecticut, Idaho, Indiana, Michigan, Oregon, New Mexico and Utah, said Dr. David Acheson, chief medical officer for the FDA's food safety center, at an evening news conference. Wisconsin has been hit the hardest, with at least 20 cases, including the patient who died.

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"It's increasing by the day," Acheson said of the tally of cases.

One possible case is under investigation in California, where the bulk of the nation's spinach is grown. California health officials said they were working with federal authorities to investigate any possible sources of the outbreak in this state, a spokeswoman said.

For unknown reasons, the infections have been associated so far only with spinach that has been bagged, not unpackaged greens. That raises the possibility that contamination occurred either in fields dedicated to packaged spinach or during processing.

The FDA announcement came after Wisconsin and Oregon officials contacted the Centers for Disease Control and Prevention on Wednesday to report unexpected surges in E. coli cases. So far, the 49 cases identified share the same genetic fingerprint, suggesting a connection, officials said.

The number of cases, which primarily have affected adult women, could climb. Officials urged anyone with bloody diarrhea or diarrhea accompanied by severe cramps "” signs of E. coli infection "” to contact a doctor. When ingested, the suspected strain of E. coli, O157:H7, can produce powerful toxins and, in rare cases, trigger a serious condition that can lead to kidney failure.

Although officials recommended avoiding fresh bagged spinach altogether, boiling the greens probably would kill the E. coli.