Fire officials say they know what started a warehouse fire in the eastern Phoenix suburb of Gilbert that generated thick black smoke that could be seen for miles.

The Gilbert Fire Department says that the blaze was set off by a "carelessly tossed cigarette," most likely from an employee.

The blaze ignited Monday afternoon and involved mostly materials in a backyard storage area behind Farnsworth Wholesale, a plumbing wholesale company.

Dozens of fire trucks from Gilbert, Tempe, Chandler and Mesa were at the scene and an aerial tanker sprayed water for about an hour.

State environmental officials allowed nearby residents to return by 7 p.m. Monday but told them to minimize exposure outdoors because of smoke fumes.

A spokesman for Farnsworth says the facility will likely be closed for several months.

Eighty employees were inside at the time the fire started, and everyone escaped safely. The company has transferred them to other locations.

Video from Christopher Kinghorn

Photo above by John McJunkin