CLIFTON, N.J. (CBSNewYork) — Five students, all between ten and eleven years old, were detained after allegedly plotting to ’cause damage’ or ‘detonate a device’ at Clifton High School, authorities said.

According to authorities, teachers and administrative leaders found suspicious letters from the students claiming they were going to ‘attack’ the school. The school then notified the Clifton Police Department.

Police found a ‘suspicious device’ when investigating the threat, but the device was later proven to be non-explosive, authorities said.

The five detained children have since been released to their families, according to police. Their names will not be released because of their ages.

According to police, this was an isolated incident and there are no current safety issues at the school.

CBS2 has reached out to the Clifton Public Schools Superintendent.