A cigarette likely sparked the Gilbert warehouse fire that caused $8 million in damage and sent a plume of black smoke into the sky for hours on Monday.

Eyewitnesses along with video from news helicopters and surveillance cameras helped investigators pinpoint how the fire began in an outdoor storage area at Farnsworth Wholesale, Gilbert fire Capt. Mark Justus said.

The footage made it clear the blaze started in a stack of fiberglass bathtubs that were packaged in cardboard. The area was open to employees and customers, but Justus said it wasn’t immediately clear who was smoking. Investigators will likely stay on the case for days to formalize what is now a preliminary finding.

The fire triggered an unusually large response — 200 firefighters, 30 fire engines and 10 ladder trucks — and an unusually large investigation. Justus said an investigative team formed from the responding agencies of Mesa, Gilbert, Chandler and Tempe.

“Usually the investigations are handled by the local agency,” he said. “But this thing was so big that they just continued.”

As many as 50 firefighters remained Tuesday morning, using heavy equipment to access hot spots. Crews left the scene about 3 p.m. Tuesday — 24 hours after the blaze was reported south of Baseline Road, between Country Club Drive and McQueen Road. No one was injured in the fire.

The fire caused multiple explosions as it reached propane tanks. Plastic and fiberglass caused the black smoke that could be seen across the Valley. Some residents near the fire were evacuated briefly as state environmental officials tested the air quality.

Firefighters kept the blaze confined to that location despite strong winds, Justus said. While the building was heavily damaged, crews prevented it from destroying everything.

“The offices sustained water damage, but not to the level you would think,” Justus said.

Farnsworth Wholesale has a showroom and warehouse in Gilbert. It also operates in Mesa and Peoria.

Contact writer: (480) 898-6548 or ggroff@evtrib.com