Eleven New Jersey schools were placed on lockdown or evacuated for part of the day Tuesday after officials received threats about mass shootings and bombs, authorities say.

Schools in Tenafly, Leonia, Bergenfield, Teaneck, Garfield, Clifton, Fair Lawn, Englewood, Hackensack and Sayreville were either evacuated or placed on lockdown for at least some time Tuesday morning after schools received the threats, authorities say. But the more than 10,000 students who were affected were able to continue their days once officials determined the threats to be part of a hoax.

In each instance, officials say someone claiming to have planted a bomb or was planning mass shootings, authorities said.

All of the threats sounded similar to an automated robotic call and had been left as voicemail messages, authorities said. According to Bergen County Sheriff Michael Saudino says the messages appear to have been routed through Bakersfield, California.

In Bergenfield, students were forced to huddle together outside on a day with wind chills in the teens while tactical crews swept through the schools for signs of a threat. They spent more than an hour in the cold -- some students without jackets -- before walking to a nearby middle school.

"Everyone was cold," said Amber Morales, a senior. "Teachers were handing out blankets; people were sharing sweaters."

Most of the students lost an entire day of instruction due to the threats, officials said. Hundreds of students also decided to head home out of fear.

Schools in several other states in the Northeast and Iowa also reported threats Tuesday morning. It was not immediately clear if the threats were linked, authorities said.