(CNN) A 74-year-old man has been discovered alive five days after his senior living apartment complex in Washington, DC, caught fire -- but nobody knew he was missing.

Mayor Muriel Bowser told reporters the man was found by crews working to determine whether fire marshals could safely enter the building.

He was taken to a hospital with injuries that were not life threatening, Bowser said.

Structural engineer Allyn Kilsheimer said crews were going from unit to unit Monday when they found the man trapped in his apartment.

Some of the doors had swelled because of the fire's heat, Kilsheimer said, which prevented them from being opened. Workers had to use crowbars to pry some doors open.

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