This spring F.D.A. investigators traced the virulent strain of E. coli found in romaine to the growing region of Yuma, Ariz. But the last romaine lettuce was harvested there on April 16 and the shelf life for lettuce is about three weeks. According to the C.D.C., the onset of the last reported illness was on May 2.

Federal investigators are still looking for the precise source of the virulent strain. Dr. Stephen Ostroff, the F.D.A.’s deputy commissioner for foods and veterinary medicine, said researchers were looking for patterns — in water supply, harvesting equipment, even shared work crews. “There are a lot of ways this could have happened,” he said. “The easy answers don’t explain this. We have to look at something potentially different.”

In 2006, after the danger from a toxic strain of E. coli tied to spinach that sickened 205 people across 26 states had passed, researchers hunted for another seven months. They finally concluded that the bacteria was traced to river water, cattle feces and wild-pig feces on a California cattle ranch scarcely a mile from a spinach field.

During the latest outbreak, researchers for the F.D.A. and the C.D.C. warned consumers, including institutional purchasers like schools and restaurants, not to buy romaine unless they were certain that it did not come from Yuma — a near impossibility, because individual bags of lettuce are almost never readily sourced and tracked.

The agencies’ epidemiological experts, relying on state health workers as well, extensively interviewed patients and tried to follow the fading trail of the supply chain. But critics noted that food safety monitoring laws, authorized by Congress in 2010, have still to be fully put in place, hampering the efforts of researchers to swiftly find and recall fresh produce.