Health authorities hunting the source of a nationwide E. coli outbreak are focusing on nine California farms after discovering what could be a crucial clue: an opened bag of spinach left in the refrigerator of someone sickened by the bacteria.

The bag of tainted Dole baby spinach is the "smoking gun" that has allowed investigators to zero in on three counties in California's greater Salinas Valley, said Dr. Mark Horton, the state public health officer. Authorities also were checking processing plants, Horton said.

Officials said consumers still shouldn't eat bagged spinach, even as they closed in on the source of the bacteria as likely somewhere in Monterey, San Benito or Santa Clara counties.

The bag of fresh spinach that tested positive for E. coli was found in New Mexico, and other bags recovered elsewhere in the country also were being tested.

"It's certainly premature to say only this bag is going to test positive," said Dr. David Acheson of the Food and Drug Administration's Center for Food Safety and Applied Nutrition. "There are others in the works."

New Mexico Department of Health officials confirmed the tainted bag of spinach was found after a person who ate some of the leafy greens became one of 146 people in 23 states sickened by the outbreak. One person has died.

The spinach tested positive for the same strain of E. coli linked to the outbreak, Acheson said. Dole is one of the brands of spinach recalled Friday by Natural Selection Foods LLC of San Juan Bautista, Calif.

The tainted greens — conventionally grown spinach and not organic — came from one of the farms that supplies spinach to Natural Selection, said Samantha Cabaluna, spokeswoman for Natural Selection.

Government and industry officials were working on how to allow spinach grown elsewhere back on the market, Acheson said.

New Jersey Democratic Sens. Robert Menendez and Frank R. Lautenberg urged the FDA to assure the public spinach grown in their state was safe.

"As the nation's fourth-largest spinach producer, spinach farming is a multimillion-dollar industry for the Garden State," Menendez said. "That is why we are imploring the FDA to move quickly in identifying the source of the infected spinach."

Investigators began visiting farms in the Salinas Valley on Tuesday, seeking signs of past flooding or cases in which contaminated surface areas had come into contact with crops. They also were looking for potential sources of bacteria inside packing plants.

California produces 74 percent of the nation's fresh spinach crop. The Salinas Valley accounts for roughly three-quarters of the state's share, and it has been the focus of the investigation. The area has links to both Natural Selection Foods and a second company that also recalled fresh spinach products, River Ranch Fresh Foods of Salinas.

A third company, RLB Food Distributors of West Caldwell, N.J., has recalled Balducci's and FreshPro brand spinach products distributed to East Coast states because some of the spinach could have come from Natural Selection Foods.

Arizona and Colorado on Wednesday joined the list of states reporting E. coli cases. The others are California, Connecticut, Idaho, Illinois, Indiana, Kentucky, Maine, Michigan, Minnesota, Nebraska, New Mexico, Nevada, New York, Ohio, Oregon, Pennsylvania, Utah, Virginia, Washington, Wisconsin and Wyoming. Wisconsin has reported the most cases, as well as the lone death.

Among those sickened, 71 percent were women. Among those victims who could provide a date, they reported falling sick between Aug. 19 and Sept. 5, according to the Centers for Disease Control and Prevention.

New Mexico's public health laboratory isolated E. coli from the bag of opened spinach and then completed "DNA fingerprinting" tests late Tuesday. State and federal officials then matched it to the strain of the bacteria — E. coli O157:H7 — implicated in the outbreak.