FITCHBURG, Wis. (AP) -- Madison-area firefighters think they know what started a major apartment fire that caused an estimated $3.3 million in damages.

Fitchburg authorities believe someone put a hot charcoal grill in an outdoor storage shed on a second-floor balcony.

Residents arriving home from fireworks shows on Friday said they heard an explosion about midnight. They reported seeing flames coming from the balcony.

Fifty-two-year-old resident Darrell Reynolds says people ran through the building banging on doors and saying the fire was no false alarm. The visually impaired man says he grabbed his shoes and got out.

An American Red Cross spokesman says about 40 people have been affected, most of whom lack renter's insurance.

The apartment building has 26 units.

(Copyright 2008 by The Associated Press. All Rights Reserved.)