Firefighters responded to the second-alarm structure fire at Telegraph Road and Pioneer Boulevard at about 5:30 p.m.
Whittier police and the Santa Fe Springs Fire Department worked to put out the blaze. Both streets were expected to be closed for several hours, Whittier police said.
One man was found dead inside the building, according to Whittier PD. No other injuries were reported.
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The cause of the fire was unknown as the investigation was ongoing.