Police in Vermont investigating the sudden death of 24-year-old actress Sophie Nyweide say she was found lifeless on a riverbank … and they’re looking into a range of possible causes, including foul play.

Bennington Police tell TMZ … Sophie’s body was found not far from a high school in Bennington around 4 AM on April 14, and she was declared dead at the scene.

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We’re told her body was near a lean-to — a makeshift structure made from downed trees — and nothing is being ruled out at this point in terms of a cause of death.

Police say Sophie’s autopsy is still pending, and will include a toxicology report.

We’re told a man was with Sophie when she died, and he is cooperating with cops … though cops add he’s not considered a suspect or person of interest.

Police tell us there were other people hanging out with Sophie at the lean-to before her death … and cops are trying to track them down for interviews.

We’re told Sophie did not live in the lean-to, but this was not her first time visiting the structure.

Sophie Nyweide’s mom previously told us she was using drugs.

She was 24.

RIP