The entire building was searched following the 11:07 a.m. lockdown, Washington Township Police Chief John Calamari said.
The search confirmed there wasn't a threat of any kind and the lockdown was stepped down to a shelter in place.
The system malfunctioned a second time after being reset, the chief said.
An early 12:50 p.m. dismissal allowed technicians to investigate the cause, Schools Supt. Jill Mortimer said.
"The company we use" for the alarm system was summoned to assist, she added."I know this was an extremely stressful experience, and for that I am sorry," the superintendent said.
Calamari, meanwhile, thanked police from Westwood, River Vale, Hillsdale, Emerson and Ridgewood and the Bergen County Regional SWAT team for their rapid response.
He also said students and staff "followed all appropriate protocols and should be commended for their professionalism and cooperation....The school is now considered safe and secure."
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