It has become all-too-common in cities and towns across Massachusetts: An automated telephone call warns there is a bomb inside a school, forcing early morning sweeps and searches, delays, and sometimes evacuations.
On Monday, for the third time since January, the FBI, State Police, Fire Marshal's Office and local police were deployed to investigate robocall threats at 21 schools, bringing the number of affected schools to at least 45 this year.
While no explosives have been found, educators say the threats disrupted the schedules of thousands of students and leave them on edge. For some schools, the alarms are becoming almost a monthly occurrence, but officials say they cannot be taken lightly amid a heightened concern about school violence nationwide.
"This is a strange new world we live in," said John O'Connor, superintendent in Tewksbury, which was threatened for the third time this year on Monday. "It's far more complex than it's ever been, and it's getting to be more and more of a challenge to maintain the integrity of the schools' instructional program and learning when we're dealing with issues of security.''
Ed Davis, the former Boston police commissioner, believes the threats could be coming from abroad. He said the perpetrators use computer programs that allow them to disguise their location, making them difficult to track.
"There's no easy way to track them down," said Davis, now a private security consultant. "But a lot of times people who do this will make a mistake along the way, and that's what you have to hope for."
Threats that targeted Rhode Island schools earlier this year probably came from Russia, authorities said.
Ken Trump, a Cleveland-based national consultant on school security, said the robocalls fit the illegal practice of "swatting," where computer- and Internet-savvy hackers phone in threats using false numbers to create chaos.
Trump said that robocall threats are escalating around the country, and he cited the one in December that prompted Los Angeles to shut down 900 schools.
Police in New York dismissed a nearly identical threat as a hoax.
"This has all of the marks of a swatting incident: similar threats in a short period of time, with multiple targets with computer-generated robotic voices," he said.
Many communities handled Monday's bomb scares internally, using police and fire and school personnel — who had been trained over the last couple of years by the Department of Fire Services — to determine the threat.
In addition, the State Police Bomb Squad was dispatched to some schools, said Jennifer Mieth, spokeswoman for the Department of Fire Services.
She said the investigation is being led by the FBI.
FBI spokeswoman Kristen Setera declined to provide details about the robocall investigation.
The FBI, State, and local police patrols, K-9 units, and bomb squads responded to many of the schools, State Police spokesman Dave Procopio said.
On Monday, schools began receiving the threats just after 7 a.m. The messages typically shield the identity of the caller — the same type of automated technology that politicians and telemarketers use to disseminate messages.
Phone calls were received at high schools in Ayer, Billerica, Brighton, Brookline, Charlestown, Cambridge, Hopkinton, Hudson, Leominster, Lexington, Lynn, Marlborough, Melrose, Medway, Stoneham, and Tewksbury.
In addition, threats were called in to Boston College High School, Melrose Middle School, Shawsheen Valley Regional Vocational Technical High School, and Newton North and South high schools.
Somerville City Hall also received a call.
At Newton North, which has received three threats since January, students were kept in their first-period class while police searched the building. Superintendent David Fleishman called the threats "frustrating" and praised the students for staying calm.
"The fear at the end of the day is that if this happens on a regular basis then kids won't be in school," said Fleishman.
In Billerica, hit by threats in January and February, a heavy police presence remained at the school Monday after law enforcement swept it for explosives.
"Please know that we treat the safety and security of our students as our most important responsibility," Billerica Superintendent Timothy G. Piwowar wrote on the school district's website. "As such, although no bomb has ever been found in a Massachusetts school after a communicated threat, we take each threat seriously."
Many schools termed the robocalls a low threat and allowed students to stay in the schools, while police searched for explosives. Like Tewksbury, Melrose High School and Middle School went into a temporary shelter-in-place and later resumed classes.
Boston Public Schools spokesman Daniel O'Brien said that Charlestown High and Brighton High immediately notified police after receiving threatening messages at around 8 a.m.
"Both schools were placed into safe mode for about 90 minutes while police investigated. Classes continued but students were not permitted to move around the building," O'Brien said.
Map of confirmed schools
Globe correspondent J.D. Capelouto and John R. Ellement of the Globe staff contributed to this report. Steven A. Rosenberg can be reached at email@example.com. Follow him on Twitter @WriteRosenberg.