SAN FRANCISCO (CBS SF) — Two residents were injured early Thursday morning as a two-alarm fire swept through a San Francisco Excelsior District duplex, officials said.

San Francisco fire spokesman Lt. Jonathan Baxter said crews responded to reports of a fire in the 500 block of Naples St. at 11:50 p.m.

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“Firefighters arrived on scene quickly, observing smoke and flames,” Baxter said. “(The fire was) quickly elevated to a second alarm due to the adjoining buildings.”

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Firefighters raced into the burning structure and rescued a resident who was transported to the hospital in good condition. A second resident also was taken to the hospital and expected to survive.

It took 70 firefighters until 1:21 a.m. to bring the fire under control and prevent it from spreading to other homes in the densely packed neighborhood.

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The cause of the fire was under investigation.