From The Fairfax News, this article:

Investigators say an accidental fire caused by improperly discarded smoking materials destroyed a Herndon townhouse Friday night.

Fairfax County Fire and Rescue Department units arriving at the home at 2542 Peter Jefferson Lane, in the McNair Farms West subdivision at 11:55 PM found heavy smoke and fire coming from the rear and extending up the eaves to the roof area of the three-story end unit townhouse.

The Incident Commander struck a second alarm bringing approximately 60 firefighters to the incident scene. Firefighters conducted an interior and exterior fire attack and brought the fire under control in approximately 15 minutes.