The blaze was reported at 12:30 a.m. at Parallel Processing in the 900 block of East 136th Street in the Mott Haven section.
Eyewitness News is told the flames spread quickly in the plant, while workers were still inside.
The fire went to three alarms before firefighters were able to get the upper hand.
Heavy smoke hindered efforts to fight blaze, and solar panels on the roof also made it difficult to vent the fire.
"The amount of smoke, it was very difficult to see," FDNY Battalion Chief John Ricketts said. "We had to pull the members of the fire department out for a while until we could locate where the fire was."
The situation was finally declared under control after about three hours.
All the employees made it out safely.
Five firefighters were treated at the scene for various injuries described as minor.
Four of them were taken to area hospitals for further treatment. All are expected to be OK.
The fire marshal determined the fire to be electrical in nature and deemed it accidental.
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