The U.S. Centers for Disease Control and Prevention says four organ transplant patients who developed mold infections at the University of Pittsburgh Medical Center likely got it from time spent in a “negative pressure” room normally reserved for those who already had infections.

Three of those four transplant patients later died.

UPMC has said it hasn’t been conclusively determined that the mold led to those deaths, although the attorney for one heart transplant patient’s family has since said an autopsy report lists the mold infection as her primary cause of death.

The negative pressure room was used as an overflow area when the intensive care rooms for transplant patients were filled. It’s designed to keep air from escaping, so a patient’s infection doesn’t spread.

The CDC says airborne mold may have moved into the room from other areas.