Authorities would not disclose the specific nature of the threats but said many came via automated telephone calls. FBI investigators are assisting State Police with "identifying the source of the threats," a bureau spokeswoman said.

The threats, which followed similar warnings of violence made against schools in seven Massachusetts communities Friday , were determined to be unfounded but caused widespread disruptions, forcing bomb squads to conduct sweeps and spurring some schools to close as a precaution.

At least a dozen schools across Eastern Massachusetts received bomb threats Tuesday. The spate of alarming messages led to lockdowns, evacuations, and an investigation by state and federal authorities.


In Arlington, the town's public high school was closed after an automated call threatened that someone would detonate a bomb hidden in a backpack inside the school, then shoot students as they fled, the town's police chief, Frederick Ryan, said.

"Given recent threats to many regional schools, we chose to act with a great deal of caution in investigating the threat," Ryan said. "We are actively working with our law enforcement partners to thoroughly investigate."

Arlington's superintendent, Kathleen Bodie, acknowledged that such threats can be alarming.

"We recognize that incidents like this may upset students, so our guidance department will be available on Wednesday to meet with anyone in need," she said in a statement.

Arlington Catholic High School received a threat last week, but Ryan said it did not come from a robocall but from someone who disguised their telephone number when they called.

Michael Knight, an agent and spokesman for the Bureau of Alcohol, Tobacco, Firearms, and Explosives' Nashville Field Division, said computer-generated programs can be used to make multiple threats while shielding one's identity, much like robocalls from telemarketers. Despite the broad nature of these threats, each one is taken seriously, Knight said.


Tuesday's threats extended well beyond Massachusetts.

At least nine schools in New Jersey received bomb threats Tuesday, leading to evacuations and lockdowns. None of the threats was considered valid.

The Delaware State Police said it was investigating threats at three schools that received computer-generated phone calls around 9:30 a.m. In one case, the caller said he was armed and on the roof of the school and threatened harm to the students and faculty. Students were immediately placed on lock-down.

In New Hampshire, police reported two bomb threats to high schools, one written on a boys' bathroom wall and the other through an automated call.

The high school in Farmington, N.H., was evacuated around 11 a.m. after the threat was found in the bathroom, Police Chief John Drury said. The State Police bomb squad found no explosive device.

In Portsmouth, N.H., high school students were kept inside classrooms while police investigated an automated threat.

Many Massachusetts schools remained open while police assessed the threat, and some districts quickly determined the warnings were not credible.

In Billerica, where a threat was made against schools throughout the district, officials decided against evacuating after speaking with districts that had received similar threats within the past week. Still, each building was swept for explosives as a precaution.

"Dismissal/evacuation was not warranted, and if done when not needed, rewards threatening behavior and encourages more threats," district officials wrote in a message posted on Twitter.

In Taunton, officials determined an automated bomb threat was not credible after comparing it with other robocalls.


State Police also confirmed bomb threats were made Tuesday against schools in the following communities: Groton, Weymouth, Plymouth, Waltham, Ayer, Tewksbury, Newton, Swampscott, Boston, Kingston and Westford (Nashoba Valley Tech).

On Friday, threats were made against schools in Falmouth, Bourne, Mashpee, Plymouth, Weymouth, Arlington, and Boston (Boston College High School). Nothing hazardous was found at any of those schools after recorded threats that a device "would explode in the very near future," a State Police spokesman said.

Weymouth received threats around 8:15 a.m. to Weymouth High School, Abigail Adams Middle School, and Maria Weston Chapman Middle School, Superintendent Kenneth Salim said.

The buildings were placed in "stay-put mode" as school workers and police determined it was a low-level threat, Salim said.

"Weymouth police then conducted a sweep of the buildings with a physical walk-through and a follow-up search with K-9 units and there was no indication of a threat," he said.

Parents were contacted throughout the morning, and schools later resumed their regularly scheduled day, Salim said.

A look at the schools and districts where threats are occurring:

John R. Ellement of the Globe staff contributed to this report, and material from The Associated Press was used. Peter Schworm can be reached at peter.schworm@globe.com. Follow him on Twitter @globepete.