MADISON, Wis. -- The Madison Fire Department said Wednesday that investigators have determined a two-alarm fire at an apartment building on Madison's West Side on Saturday night was caused by discarded smoking materials on a second-floor balcony.

Firefighters responded to the fire at an apartment building at 1040 North High Point, and they arrived to find flames through the roof of the two-story, 28-unit structure.

Internal alarms were sounding and all the occupants had evacuated on their own.

Officials said the fire quickly went to two alarms. Knockdown took almost an hour as crews battled flames in the roof and inside exterior walls.

Fire officials said one crew of firefighters narrowly escaped injury as they approached the building when a propane tank on another balcony exploded.

Three people, two adults and an infant, were transported to an area hospital suffering from smoke inhalation.

Madison Fire Department spokeswoman Lori Wirth said damage to the two-story structure is now estimated at $800,000, with another $250,000 in losses expected for contents. Officials said none of the 28 units in the building on North High Point Road can be occupied in their current state.

Wirth said the fire taxed the city's firefighting resources. She said that when a second report of a fire came in on the city's East Side Saturday, there were only two crews left who were not involved in active firefighting.

Wirth said investigators have concluded that the second fire called in as firefighters battled the High Point fire was also caused by discarded smoking materials. That fire, at 516 East Lakeview Ave., began near a rear deck when an occupant discarded a cigarette that ignited dried brush nearby. Wirth said damages in that fire are estimated at $10,000.

No one was injured.