ST. PAUL — Minnesota health officials said they are unable to identify the source of an E. coli strain that killed a 3-year-old Wright County girl and sickened her older brother.

The state Health Department said Monday it has completed its investigation but was unable to identify a single source of E. coli.

The parents of Kallan Maresh said the bacteria toxin attacked their daughter’s kidneys and neurological system and led to her death. Kallan and her 5-year-old brother, Kade, contracted the bacteria July 9, sending them into kidney failure.

The Health Department said disease investigators considered all known potential sources of E. coli, including farm animals. But officials did not find a connection with a petting zoo the children visited, despite extensive testing of the animals.

No additional cases have been reported.