A leftover sample of cooked ground beef prepared for tacos had pieces of meat that were still red in the middle, allowing any E. coli bacteria to survive the cooking, said Dr. John Kobayashi of the state Department of Health.
However, neither state nor U.S. Department of Agriculture investigators could find E. coli bacteria in leftover cooked beef or frozen beef from the same lot.
"This does not rule out meat as a source," Kobayashi said. "(E. coli) is difficult to isolate from food, meat in particular."
The school cooked 108 pounds of hamburger in one kettle, then divided it up to send to the high school, middle school and elementary school in Finley, health officials said.
Thermometers used to test the meat read as high as 190 degrees, school officials said. That's 30 degrees over the minimum recommended temperature.
But the state recommended the school stop cooking such large batches of meat.
"When you cook that much, there can be inadequate mixing of the meat," Kobayashi said. "There can be very hot temperatures at some locations and not at others."
The way the beef was handled for the different schools also explains why only elementary school students fell ill, he said.
Meat for the high schools and middle schools was placed in smaller pans on stoves to keep it hot until it was served to students. That additional cooking could have been enough to kill E. coli bacteria, state officials said.
However, meat sent to the elementary school received no additional cooking. It was kept warm on a steam table that maintained the temperature but did no additional cooking, said Dr. Larry Jecha, health officer for the Benton Franklin District Health Department.
The Finley School District traditionally has had a good record for compliance with food safety procedures, Jecha said. It had cooked hamburger the same way many times before without incident.
However, the beef served Oct. 6 was leaner than what the school usually cooked, said Mary Ferluga, supervisor of the food safety program for the county health department.
Leaner meat would not have cooked down as much as that with more fat, she said. A larger amount of meat would have had to be cooked longer or at a higher temperature to get done, she said.
The state also considered but ruled out the possibility that lettuce served Oct. 6 and 8 could have been the cause of the outbreak. No lettuce was left from the meals to test, but there was little evidence to point to it other than lettuce irrigated or washed with contaminated water has caused outbreaks before in other places.
The packaged lettuce from the same shipment had been widely distributed throughout Benton County and in Oregon.
"We would have expected to see cases out of the Finley area" if the lettuce had been contaminated before it was packed, Kobayashi said. Little handling was done after packages were opened at the school district, making contamination by an employee there unlikely, he said.
For now, the Finley School District has stopped serving ground beef in its school lunch program. If it resumes serving it, it's considering buying precooked hamburger that's prepared for institutions, county health officials said.
The district's taken several other steps, including ordering high-tech thermometers, requiring all elementary school students to wash their hands before lunch and providing more training for food service workers.
"It's probably one of the safest places to eat in the county," Jecha said.
None of the elementary school children remains hospitalized, although some are not attending school full time yet and one boy who received blood transfusions still has high blood pressure.
School and health officials are most concerned about a 12th child, who is suspected of being contaminated with the bacteria after playing with students in her neighborhood and family who ate the lunch.
On Tuesday, Faith Maxwell, 2, spent her 11th day on dialysis at Children's Hospital in Seattle after developing a kidney complication. No tests have confirmed she was contaminated with the E. coli bacteria, but symptoms of her blood and kidney problems often are caused by the bacteria, Kobayashi said. Not all test results are completed.
The longer her kidneys are not functioning, the greater the chances she will have long-term problems, said Bill Marler, a Seattle attorney representing the Maxwell family and three other families whose children were hospitalized.
None of the other children required dialysis.
Marler said he is concerned by reports the school did not cook the hamburger enough. "But ultimately, the problem was in the meat," he said.
"The families do not want us to focus our energies on the school district," he said. "My focus is on the supplier and the manufacturer."
However, if the supplier and manufacturer are sued, they likely will file a cross claim with the school district, Marler said.
The meat served Oct. 6 came from a USDA commodities program. It most likely was processed in October 1997 in Nebraska and kept frozen until served a year later in Finley.
Helmut Blume, a USDA district manager, said Tuesday that his office had not been notified of the results of the state investigation, so he was not prepared to comment.