DANVILLE, Pa. (AP) — A Pennsylvania hospital says it has discovered the source of bacteria that infected at least eight premature infants, killing three of them.

Geisinger Medical Center in Danville says the process it was using to prepare donor breast milk led to the deadly outbreak in the hospital’s neonatal intensive care unit.

Infection control specialists used DNA testing to trace the Pseudomonas bacterium to equipment used to measure and administer donor breast milk. Geisinger says it has since switched to using single-use equipment. Hospital officials stress the milk itself was not the source of the exposure.

Chief Medical Officer Dr. Edward Hartle says in a statement that Geisinger extends its “sincere apologies” to the affected families.