Woodbridge police responded to Amity Regional High School Monday morning after some students and staff received an email about a possible threat at the school.
School administrators made the call to shelter in place, which limits movements throughout the school, police said.
Officers could be seen blocking off some entrances to the school.
The email was also circulated among students on social media.
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Administrators did not dismiss students for the day, but NBC Connecticut saw many parents picking up their children from school early.
"Just frustrated that the school day was so disrupted," said Susan Cassidy, a parent of a student at Amity High. "I know the school did the best they can and it’s a tough situation and there’s only so much information that can be given out but when parents have a little bit of information and not all the information you tend to get crazy behavior."
Police determined there was no credible threat to the school. They are working with state and federal authorities to determine who sent the threat.
There will be an additional police presence at Amity High School Tuesday.