The fire broke out at 6:30 p.m. at the Park Plaza Diner on Cadman Plaza in the Brooklyn Heights section.
The kitchen staff says they were getting ready for a catering job when the fire began.
When firefighters arrived, there was heavy smoke coming from the roof.
The diner was evacuated until the fire was brought under control.
One firefighter was hurt battling the smoke and flames, that spread through the ventilation to the roof.
Investigators determined that the fire was sparked by grease coming from a duct inside the diner.
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