Snohomish Health District officials are investigating at least six confirmed cases of E. coli.

Dr. Gary Goldbaum says that while at least six cases have been confirmed, health officials still don't know what caused people to become ill.

The public heath agency alerted doctors Friday afternoon that it was investigating a possible cluster of E. coli cases.

It asked physicians to test patients who have symptoms that may indicate they have the bacterial infection. The agency sent an e-mail alert to physicians and area health care groups.