Students at Sandy Hook Elementary were evacuated and taken to a nearby school where they were reunited with their parents.

A bomb threat was called into the school in Monroe at about 9.30am.

Lieutenant Brian McCauley says the school building and grounds were checked and no evidence of danger was found.

Investigators are trying to track down the origin of the threat.

The town of Monroe opened the school to students and teachers from neighbouring Newtown, after a gunman shot and killed 20 children and six adults at Sandy Hook Elementary in December 2012.

The parent of one child killed in that attack praised the quick response to today's threat.

"We are first grateful to hear that all children and staff were physically unharmed," Jimmy Greene told the Connecticut Post.

"I am thankful for the quick response and transparent communication of the superintendent and the staff of Newtown Public Schools."

Mr Greene added that the threat "clearly indicates the need to reach those people who choose to commit such cruel and thoughtless acts".

The Newtown school was demolished in October 2013.