One patient was killed and three others suffered slight injuries in a fire at Durham Regional Hospital in central North Carolina early Tuesday.

Firefighters were called to a report of an explosion on the sixth floor of the hospital around 2:15 a.m., Durham Fire Department spokeswoman Sierra Jackson said. The firefighters discovered there had been no explosion and the fire had been extinguished by the hospital sprinkler system.

The cause of the fire was still under investigation, Jackson said later Tuesday morning.

Hospital officials were still investigating exactly where the fire occurred and how, said Katie Galbraith, hospital chief of operations.

The hospital was operating normally several hours later.

The sixth floor of Durham Regional is a unit operated by Select Specialty Hospitals, a company that provides care for critical and complex cases that require more attention than conventional patients, the Raleigh News and Observer newspaper reported. Other patients in the 30-bed unit were moved to other parts of the hospital, which is owned by Duke University. The company did not immediately return calls for comment. 

Some other patients were moved because of flooding caused by the sprinklers.

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"They are safe and they are being well cared for," Galbraith said.

The three slightly injured patients, who had been on ventilators before the fire, were taken to the emergency room to be checked for smoke inhalation and then sent to the intensive care unit.

The names of the fatality and those injured were not immediately released.

Galbraith said the hospital staff practices for just such emergencies.

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

Durham Regional is a 369-bed acute care hospital.

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NBC News' Jim Gold contributed to this report.

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