U.S. Marines raced to the scene last week to help firefighters rescue the original victims.
On Monday, as structural teams checked the building's charred remains, they discovered the man who they say seemed in incredibly good shape.
D.C. Mayor Muriel Bowse blames the building managers, who reported the man had been accounted for.
The three-alarm fire started around 3:30 p.m. Wednesday.
Officials say 160 residents are displaced as a result of the fire.
The cause of the fire is still under investigation.
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