Witnesses didn't see a single flame or billows of black smoke rising from a warehouse on Flushing Avenue.

"There was no real fire that was visible. It must have been contained," said Jose Rueda.

But a deadly fire was brewing behind warehouse walls in Bushwick.

"I saw a bunch of firefighters being brought out in stretchers," said Roy Torres.

Firefighters got a call at 1 p.m. about a fire.

It quickly grew to a three-alarm requiring more than 170 crews on foot to knock it down.

But before they could, six firefighters were removed from the building because of injuries. All were rushed to the hospital. Lt. Richard Nappi, was one of them, and he died there.

"While operating a hose for Engine 237, Nappi felt dizzy. He was removed from the building and speaking to medics on scene. Then he suffered some sort of cardiac event," said Fire Commissioner Salvatore Cassano.

Nappi was a 17-year veteran of the FDNY.

Firefighters are investigating the fire's cause and if high levels of carbon monoxide contributed to Nappi's death.