A fire broke out on the old Norwich State Hospital property in Preston on Sunday.
Firefighters said a former residential structure was fully involved.
In a video posted by firefighters, large flames can be seen coming out of the building.
Authorities have not released details about the extent of the damage.
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The cause of the fire is under investigation.
The Norwich Hospital property is right next to the Thames River off of Laurel Hill Road. It used to be a hospital for the mentally ill.
Back in 2017, the Mohegan Tribal Gaming Authority had plans to develop the property. They wanted to create an indoor and outdoor entertainment area.