Three cases of E. coli infection since September have prompted the Clackamas County Health Department to begin testing all children and staff at a West Linn Montessori program for the potentially deadly bacteria.

In a Thursday letter to parents with children at Heart Centered Montessori, 2152 S.W. Ek Rd., the department said it continues to investigate the source of the infections. Meanwhile, health officials recommended students and employees be tested for E. coli and to avoid returning to school, swimming and participating in group activities until test results are back.

The Montessori school serves children up to age 6 and was established in 2007, according to its website. 

At least two of the three infections are of the most virulent type of E. coli, O157.H7, the letter to families said. A 4-year old girl in Lincoln County died of the same bacterial strain in September.