Princeton High School was evacuated Tuesday morning after Princeton Police received a call threatening that there was a bomb inside the school. The threat, which came from an unknown male, was called in about 10:15 a.m., and officials immediately emptied out the building and brought in bomb-sniffing K-9 dogs. No device was found.

Students could be seen sitting on the bleachers and ambling around the football field behind the building on Walnut Lane while the school was searched. Police were stationed all around the school. Students were able to re-enter the building around 12:20 p.m., according to police reports.

Last month, Ewing High School received three bomb threats in three weeks, and students were evacuated each time. Ewing police arrested a 15-year-old boy in connection with one of the incidents.

Princeton schools received several “swatting” calls last year warning of explosive devices. The threats concerned the high school, Riverside Elementary School, and John Witherspoon Middle School.

Special information sessions were held for parents and students to calm nerves regarding the incidents, which were also happening in other parts of the country from Alaska to Connecticut. Princeton’s school district was targeted 10 times.

Local police collaborated with the FBI and other federal and state authorities to try and determine who was making the threats. Some of the threats were found to be pre-recorded, while others appeared to be through Internet connections, on a computer, possibly from the gaming community.

No bombs were found at that time, and the calls were considered to be a hoax.