School officials said Wilby High School in Waterbury was evacuated mid-morning on Tuesday.
School officials said a bomb threat message was discovered inside the building, which is located on Bucks Hill Road.
Students were evacuated for about 45 minutes.
Waterbury police swept the school with the help of the fire department. No device was found.
Students were let back inside shortly after 10 a.m. Officials said the school day will continue as normal.
Police said they are investigating.
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