Durham, N.C. — Investigators said a fire that killed a patient and injured three staff members at Durham Regional Hospital early Tuesday may have started during a defibrillation procedure.

Preliminary findings by the state Department of Health and Human Services determined a spark happened during the defibrillation, igniting the blaze that broke out on the sixth floor. It was not immediately clear whether the spark was caused by equipment malfunction.

Investigators also said the hospital's sprinkler system worked properly and the staff responded appropriately to the incident.

The fire was initially reported as an explosion about 2:15 a.m., although Durham Fire Department officials said there was no evidence of an explosion when they arrived on scene.

In a 911 recording released by officials Tuesday, witnesses described a chaotic scene, saying the smoke was "overwhelming" and pleading with emergency responders for help.

"We have a fire oxygen explosion in a room where we were doing a code," one witness told a call-taker. "We have a patient in that room and other patients around. The floor is filled with smoke."

Authorities have not released the name or gender of the person who died in the fire. A statement from the hospital said the patient who died was critically ill, and that 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 for patients who have longer stays in the hospital. The unit has 30 beds. 

No patients were evacuated because of the fire, hospital officials said, but some were moved to other parts of the hospital because of flooding caused by the sprinklers. Investigators said there was water damage to the fourth, fifth and sixth floors.

"They are safe and they are being well cared for," said Katie Galbraith, hospital chief of operations. She said the hospital staff practices for such emergencies.

"Our focus is on making sure people are safe," Galbraith said. "They did exactly what they're trained to do."

Three Select Specialty Hospital employees and one Duke employee were treated for smoke inhalation. Three patients in the area of the fire were transported to the hospital's intensive care unit to continue receiving care, officials said.