The ATF said the cause of the 5-alarm blaze is "undetermined."
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Four people were killed when fire swept through the Barclay Friends Senior Living Community in November 2017.
RAW VIDEO: Chopper 6 video of West Chester fire
Investigators believe the fire started on a rear patio.
Lack of outdoor sprinklers, which is not required by code, and excessive winds fueled the flames.
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