Ultimately, the full measure of the outbreak will not be known. Usually only the sickest patients seek medical help. The C.D.C. estimates that for every case reported to the authorities, 20 to 30 more people fall ill from the same strain; about 128,000 Americans are hospitalized and 3,000 die each year from foodborne illnesses. In a nationwide outreach to clinicians, C.D.C. officials have emphasized that Shiga toxin illnesses should not be treated with antibiotics.

By April 13, the C.D.C. announced that 35 people from 11 states had become ill from the same strain of E. coli, now linked to romaine lettuce from the Yuma region of Arizona. F.D.A. investigators traced the sickness among a cluster of eight inmates at an Alaska prison back to whole-head romaine that had been harvested from Harrison Farms, in the Yuma area. But they could not link other cases to the same farm.

Harrison Farms is a member of the Arizona Leafy Greens Marketing Agreement, an organization of producers whose practices meet or exceed requirements established through the Food Safety Modernization Act, said Teressa Lopez, a spokeswoman for the group.

But it turns out that romaine is not romaine is not romaine.

It can be processed and distributed in many ways — chopped, cored, sold as hearts or even mixed with other greens in salad bags. The more processes, the more convoluted the trail. The scores of patients who became ill after eating romaine at restaurants had not consumed the whole-head product.

Dr. Stephen Ostroff, deputy commissioner for foods and veterinary medicine at the F.D.A., compared so-called traceback efforts to finding common points of intersection among flight paths on an airline magazine’s map. Step by step, investigators work backward from each known point of contact for a patient, sifting through menu items, individual recollections, bills of lading, distribution sites, chopping and bagging facilities, locations where lettuce is cooled, trucks and fields. It is rarely linear. Finding a needle in a haystack is a no-brainer compared with finding the source of the Shiga toxin-producing E. coli making its way around the country.

Officials say the bacteria almost certainly originated in the fecal material of an animal. But was it tilled into the soil? Found in farm animals? Did deer nibble and excrete their way at night through fields?