Durham, N.C. — A fatal fire that erupted during a defibrillation procedure at Durham Regional Hospital earlier this week was an accident, investigators said Thursday.

Durham Fire Department investigators said they are still working to determine the source of ignition in the blaze that broke out shortly after 2 a.m. Tuesday in a room on the sixth floor. One patient died and three employees suffered smoke inhalation.

Preliminary findings indicate a spark happened during an attempt to revive the patient, who was critically ill. The identity of the person has not been released, and hospital officials said the timing and cause of death will be determined by the state medical examiner.

The sixth floor houses Select Specialty Hospital, a separate facility that specializes in long-term acute care. The fire was initially reported to 911 as an explosion, and callers described a chaotic, smoke-filled scene.

Investigators have said the sprinkler system functioned properly and hospital employees responded appropriately to the incident.