BOSTON (CBS) — Schools in more than a dozen towns across Massachusetts received bomb threats Monday morning.

Massachusetts State Police said that the threats came in via robocalls and that the State Police Bomb Squad was responding to some of the communities that received the threats “where appropriate.”

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Some of the affected communities included Ayer, Billerica, Boston, Brookline, Cambridge, Hopkinton, Hudson, Leominster, Marlboro, Melrose, Tewksbury, and Wakefield.

The specific nature and the source of the threats are not yet known.

Hudson Public Schools Superintendent Jodi Fortuna said the number of threats is unprecedented.

“I can’t even speculate on why somebody would be doing this,” Fortuna said. “It’s unfortunate because in a lot of ways it takes very valuable public safety resources away from other things.”

In Boston, police said robocall bomb threats began arriving around 7:45 a.m. at Brighton, Charlestown, and BC High Schools. After a search, all three schools were deemed safe by Boston Police.

In Cambridge, police found nothing after searches but said additional officers would be present throughout the rest of the school day.

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Brookline Police said that Brookline High School had been evacuated, but students were let back inside the school a short time later.

State Police said each community was following protocol in assessing the threats and responding as needed at the local level.

The FBI said they had offered assistance to state and local authorities, but that they were not directly responding.

A similar rash of bomb threats made to schools via robocalls happened earlier this year. In those incidents, no hazardous materials were found at any of the affected schools.

WBZ-TV security analyst Ed Davis said that last round of “robo-threats” was found by the FBI to have come from Russia, and that the caller or callers had used a “masking” program to hide their identity.

Cybersecurity expert Robert Siciliano, of Hotspot Shield, showed WBZ how easy it is to find robocall services.

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“They are facilitating the process of you changing your caller ID to mask who you are,” Siciliano said. “And you just provide them with a list of phone numbers you want to call and a script and you just hit a button and all of that takes place right in front of you.”