Health officials in Oregon are testing several possible contaminants that could be the source of the E. coli infection that killed a 4-year-old Oregon girl this week, but they warn that the source may never be found.

Serena Profitt died on Monday in Portland after suffering from an E. coli infection for more than a week. Food Safety News spoke with her uncle on Tuesday when reporting on her death. A family friend, 5-year-old Brad Sutton, is in critical condition and on dialysis in a Tacoma, WA, hospital but was reported on Thursday to be steadily improving. The two children were playing together over Labor Day weekend and apparently shared one meal — a turkey sandwich — at a restaurant. The children also played in a pond, which has been connected to E. coli cases in the past. Both children later tested positive for E. coli O157:H7. But health officials are not ready to place blame on the sandwich. Investigation into the restaurant where it was served showed no evidence of E. coli exposure, according to Dr. David Long from the Lincoln County Health & Human Services Department. “We’ve investigated the places that the people have been in the public and so far we haven’t found any evidence that there’s a source that would be potentially dangerous to the public,” Long said at a Thursday news conference in Newport, according to KOIN 6 News. On Sept. 5, a 3-year-old girl in Washington state also died from an unrelated E. coli infection.