As the search for the source of salmonella-tainted tomatoes dragged on, federal officials announced several more cases of infection Monday.
The rare Salmonella Saintpaul strain has now caused 277 reported infections in 28 states and Washington, D.C., since mid-April and has led to at least 43 hospitalizations, the Centers for Disease Control and Prevention said.
The Food and Drug Administration eliminated New Mexico, Indiana and Baja California as the origin of the outbreak Saturday and cleared Connecticut and Washington, D.C., late Monday.
So far, the FDA has excluded 38 states, including California, and declared tomatoes grown in those regions to be safe to eat.
A cluster of nine cases, all from the same geographical region and involving similar types of tomatoes, was "the most fruitful lead to date," said David Acheson, the FDA's associate commissioner for foods, in a conference call Monday.
He confirmed Friday that the nine people contracted salmonella after eating at two restaurants from the same chain but would not name the chain or give any identifying characteristics, such as the location of the restaurants.
"There's a lot of aggressive work going on," said Acheson, who did not know how much the FDA had spent on the probe. "I'm optimistic that this cluster will help us, but as we've found before, especially with tomatoes, things can become disconnected even when you think you're almost there."
Acheson would not say whether the cases were the same nine that the Chicago Department of Public Health identified as salmonella cases linked to tomatoes eaten at a restaurant in May.
And though health officials are also following other cases, "there is no one chain of restaurants or a grocery chain that accounts for all of the cases" in the outbreak, said Dr. Ian Williams, chief of the OutbreakNet Team at CDC.
Health experts are still unsure whether the outbreak originated at a farm or a packing station.
The search has begun focusing on southern and central Florida and Mexico, though the FDA said tomato shipments from Mexico and northern Florida were safe if they were accompanied by a certificate from the areas' respective agriculture departments.
Though the FDA has stepped up its sampling of tomatoes, Acheson said that none of the tested fruit came back positive for salmonella. Williams said the CDC has heard no reports of salmonella infections in Mexico.
Consumers and farmers have criticized health officials for the slow pace of the probe, which the FDA says is unavoidable because of the wide scope of the outbreak, the shaky memories of victims and the difficulties of tracing tomatoes.
As a result, shoppers have shunned the fruit and growers have suffered plunging sales.
Consumers should continue to steer clear of any fresh Roma, plum or standard round tomatoes from states that have not been cleared, according to the FDA. Any tomatoes with an indeterminate origin should not be eaten. The agency said cherry and grape tomatoes and tomatoes still attached to the vine were fine.
Though Williams said the outbreak was still classified as ongoing, with the most recent reported illness June 5, many of the tomatoes harvested in the initial outbreak period have exceeded their shelf life and were likely no longer in circulation.