The road was closed in front of the commercial shopping strip in Sand Hill Plaza, police said.
The road will be closed for the "next eight hours," police said at about 9:45 p.m. Tuesday.
It was not clear whether the closure would affect the morning commute.
Just before midnight, Newtown police said Route 25 remained closed.
The Connecticut State Police Accident Reconstruction team was on the scene of the crash and was conducting an investigation.
Police said there would be an update Wednesday.
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