Associated Press - March 13, 2009 1:45 PM ET

TULSA, Okla. (AP) - State investigators have tested nearly 110 private water wells as a probe into last summer's deadly E. coli outbreak in a northeastern Oklahoma town nears an end.

Of the wells sampled so far near Locust Grove, 20 were positive for E. coli bacteria, but none of those samples turned up the rare O111 strain that killed one man and sickened more than 300 people.

The August outbreak became the largest in the nation for the O111 strain, and the source of the contamination remains a mystery.

Previously, investigators had pinpointed the Country Cottage restaurant in town as the source.

But last month, Oklahoma Attorney General Drew Edmondson suggested it could have been the result of pollution from nearby poultry farms, a claim the industry denies.

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