Transcript for What Causes House Fires to Rage Out of Control?

Now to a seasonal danger, house fires. Annually, an average of 905 die every year. And many are left homeless. Kendisgibbsen has the story. Reporter: A fire quickly destroying two homes. Firefighters battling the conditions. All 12 residents, escaping the flames. Across the country this week, crews kept busy with massive fires. In Maryland, up to six family members believed dead following this mysterious mansion fire. Near Pittsburgh, fire sweeping through apartment buildings. -- The fire chief telling me his crews had no chance. It turns out the materials used in modern-day homes may be fueling the fires. When smoke alarms were first introduced, people had about 17 minutes to get out of their house. Now it's down to three or four. Still ahead tonight, the mystery of the missing gra grandparents who never returned from a trip.

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