Tests to detect E. coli could not link samples of animal manure and bedding from the Oxford Fairgrounds to the bacteria that killed a toddler and sickened another in October, state officials said Friday.

The fairgrounds had been identified as the potential source of the E. coli-related illnesses.

The U.S. Centers for Disease Control and Prevention tested samples of manure and bedding from the Oxford County Fair and concluded that they could not be tied to the children’s illnesses in October.

Further investigation of other potential causes also was inconclusive. The Maine Center for Disease Control and Prevention has closed its investigation and labeled the cause of these cases undetermined.

“The reality is that the majority of cases we investigate end up with an undetermined cause,” said Dr. Siiri Bennett, the state epidemiologist. “While we know the two children were infected by the same molecular strain of Shiga-toxin producing E. coli, or STEC, that same strain was not found in any of the samples that we tested here in Maine, or in the samples we sent to the U.S. CDC.”

Colton Guay, a 20-month-old boy from Poland, died at Maine Medical Center in Portland in early October. Another youngster, 17-month-old Myles Herschaft of Auburn, fell ill but recovered.

State officials confirmed that both boys were at the Oxford County Fair’s petting zoo.

In October, the Maine CDC was notified of the two cases of hemolytic uremic syndrome in the children. Tests confirmed that both cases had the same molecular strain of E. coli.

Samples were collected from the Oxford Fairgrounds. Four samples tested negative for E. coli, and one sample from animal pens adjacent to one barn area was positive for the presence of STEC, but the U.S. CDC’s lab tests confirmed that the sample didn’t match the strain that caused the children’s illness.

A second sample from the petting zoo area tested positive for Shiga toxins, but E. coli could not be identified. The U.S. CDC repeatedly tested the sample and no STEC was found.

E. coli can be found in food, water, unpasteurized dairy products and juices, as well as in humans and animals. Many strains of E. coli are harmless.