Firefighters were called to a group home in Willimantic just after midnight Sunday.
The fire started on the third floor of the building.
Firefighters said a resident the fire was caused by clothes that a resident left on a heater to dry.
The fire quickly spread and caused extensive damage.
There was one person in the building when the fire started, but he was able to escape without injuries.
Firefighters were called to return a few hours later when the fire sparked up again.
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