Seattle Children’s Hospital is working with the state health department and the Centers for Disease Control after it was revealed that the required procedures for cleaning and sterilizing surgical instruments at the hospital’s Bellevue Clinic and Surgery Center were not always followed.

“I understand that families will be concerned, and rightly so, but from a scientific perspective, the risk is low, which I hope that families find reassuring,’ Seattle and King County Public Health official Jeffrey Duchin, M.D. said at a press conference on August 26.

As a result of the problems with sterilization, patients who had a surgical procedure at the Bellevue Clinic may need to be tested for hepatitis B and C, as well as HIV, the hospital said in a statement.

“The risk to patients is extremely low; however, we don’t know the exact risk to each patient. We are contacting patients and offering a free blood test to those who we think may have been affected,” said hospital spokesperson Stacey DiNuzzo.Children are unlikely to have the blood-borne infections in question in the situation, said Duchin.

It has been reported that 12,000 children and their families are being notified.

Officials said that letters are expected to go into the mail on Monday, Aug. 31.  “Our patients’ safety is our top priority and we are very sorry that this happened,” said DiNuzzo. “We are investigating exactly how this failure occurred and expect the investigation to be complete this week.”

Staff reportedly noticed “debris” in packaged instruments taken off of a shelf in the clinic, prompting the investigation. Initial findings have shown that the steam sterilization step, which is the last step in the process, was working properly.

In the meantime, officials say the affected equipment has been reprocessed, cleaning and sterilization processes have been verified, and the hospital has implemented extra auditing procedures.

Correction: A previous version of this story used the name Justin Duchin. Dr. Duchin’s first name is Jeffrey, and this article has been corrected to reflect that.