Investigators will pay special attention to big packing houses or distribution warehouses that handle tomatoes from many farms and where contamination could be spread, leading to what now appears to be the nation's largest-ever salmonella outbreak from tomatoes.
"It does not mean definitively the contamination occurred on a farm in Mexico or on a farm in Florida," said Dr. David Acheson, FDA's food safety chief. "This is not just the farms that we're inspecting, it's the whole distribution chain."
A surge of newly confirmed cases moved Friday's official count to 552 illnesses in 32 states, pushing the outbreak into record territory. In 2004, government records show there were three separate tomato-and-salmonella outbreaks that together totaled 561 illnesses, the largest of which sickened 429 people.
Most of Friday's newly reported cases were people who became sick in April or May but just completed testing to prove they had the outbreak strain of salmonella.
But the latest victim got sick on June 10, meaning the outbreak may not be over.
And Texas is clearly its center, with a doubling of known cases from 131 confirmed earlier in the week to 265 as of Friday.
"These 552 may actually represent several thousand illnesses in the United States," cautioned Dr. Ian Williams of the Centers for Disease Control and Prevention.
Previous research shows that for every case of salmonella reported to the government, 30 or more people get sick but don't see a doctor or undergo confirmatory testing, he noted.
The FDA continues to urge consumers nationwide to avoid raw red plum, red Roma or red round tomatoes unless they were grown in specific states or countries that FDA has cleared of suspicion. Check FDA's Web site -- http://www.fda.gov -- for an updated list. Also safe are grape tomatoes, cherry tomatoes and tomatoes sold with the vine still attached.
The FDA already had said that central and southern Florida and parts of Mexico were suspects because they supplied the vast majority of tomatoes sold when the outbreak began in April. (In contrast, tomatoes currently being harvested in north Florida and Baja Mexico have been cleared as safe to eat.)
But Friday marked a big step in the monthlong investigation. Investigators have been tracking where the sick said they bought or ate tomatoes, and then where those retailers or restaurants in turn bought them. After a lot of frustrating dead ends, the probe finally yielded a set of clues -- a list of farms in Florida and Mexico that seem to have been at least part of that supply, plus records showing the packing houses and other distribution stops between farm and point of sale.
"A tomato that made somebody sick in Vermont has come a long way," Acheson pointed out. "A lot of suppliers and warehouses have potentially handled that tomato. ... It could be anywhere on that distribution chain where all these tomatoes were together at one point."
So FDA inspectors, working together with Florida state officials and Mexican regulators, will start at the farms and fan out to packing sheds and beyond, in hopes of finding spots where tomatoes from the farms of interest intersected.
FDA isn't aware of anyone in Mexico infected with the same strain of salmonella that is causing the outbreak, Acheson noted.
Investigators aren't stopping their interviews of the sick, in case the latest clues don't pan out. Top of that list: Texas health officials have alerted the FDA to a cluster of patients who appear to have gotten sick from the same source, potentially offering a way to narrow the hunt, Acheson said.