SAN FRANCISCO (CBS SF) — A four-alarm blaze tore through a building in the heart of San Francisco’s Castro District early Saturday morning, injuring two people.

Fire officials said the blaze was first reported at around 5 a.m. across from the historic Castro Theatre in the 400 block of Castro Street. It quickly grew from an one-alarm response to four alarms as flames billowed out of the top of the building.

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Firefighters battled to keep the flames from spreading to other buildings in the densely packed neighborhood. At least one other building suffered some damage.

Fire officials said the fire went undetected in its early stages because it started in the rear of the building and could not be seen in the pre-dawn hours from the street. It quickly made it’s way up the structure and into the attic space of at least two buildings.

Eventually, several residents called 911 to report the blaze. The residents of four apartment buildings were forced to evacuate because of the fire.

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The fire was contained at 10 a.m., fire officials said.

One person was taken to a burn center with serious injuries and a firefighter was taken to a hospital with minor injuries, according to the San Francisco Fire Department.

The blaze displaced 13 people living in the three-story building. The cause is under investigation.

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