Students were evacuated from Spotswood Elementary in Harrisonburg, Va., on Jan. 11 after a bomb threat was called into the school. No device was found in a police search. Schools in six states received threats on Tuesday. (Nikki Fox/AP)

In Delaware, the caller said he was armed, and he claimed to be on the roof of an elementary school, ready to hurt children and teachers. On Maryland’s Eastern Shore, the caller said there was a bomb inside a high school. And in New Jersey, the caller left a voicemail message for school officials saying that a bomb would be followed by a mass shooting attack.

Tuesday’s threats of violence, which affected dozens of schools in at least six states in the East and Midwest, were just the latest in a rash of threats against U.S. schools in recent weeks.

The menacing phone calls and emails have triggered police investigations and, in some cases, school evacuations. While no bombs have been found, the threats have disrupted class time and stoked fear in the wake of terrorist attacks in Paris and San Bernardino, Calif.

“One of the things we fundamentally assume is that schools are safe,” said Bradley Stein, a child and adolescent psychiatrist at RAND who helps schools work with children who have experienced trauma. “These threats create anxiety in students and in their parents,” and the effects can linger long after the threat has passed, Stein said. “It raises the possibility that schools may not be safe.”

Los Angeles Unified, the nation’s second-largest school district, decided to shutter all of its more than 900 campuses in December after receiving an emailed threat. Since then, media reports have documented a constant stream of threats against schools across the country.

There were 374 mass shootings in 2015, according the crowd-sourced database Mass Shooting Tracker. Watch this motion graphic and hear the 911 calls to get a complete picture of the human toll. (Gillian Brockell,Julio Negron/The Washington Post)

On Tuesday, the threats affected not just schools in Delaware, Maryland and New Jersey, but also in Iowa, Pennsylvania and Massachusetts.

It’s not clear whether schools are actually receiving more threats or whether it just feels that way. The U.S. Department of Education referred questions about how the number of school threats has changed over time to the FBI, and the FBI did not respond to requests for data.

It’s also unclear what role the FBI is playing in investigating the threats. Sheriff Michael A. Lewis, of Maryland’s Wicomico County, one of several Eastern Shore communities to receive multiple school threats in recent days, said that the FBI was assuming “primary investigative responsibility for these cases.” Lewis said that the FBI’s Baltimore Field Office had created a task force to catch the culprits.

Amy Thoreson, spokeswoman with the FBI’s Baltimore Field Office, said there is no such task force. “We are working with our local partners, but beyond that I can’t comment on our response,” she wrote.

Many of the threats have been made via computer-generated robocalls, according to school and law enforcement officials.

Robocalls are most often employed by telemarketers hawking vacation time-shares and political campaigns to encourage voters to head to the polls. But the technology is widely available via websites that offer robocalling services for less than a penny a minute.

School officials face the unenviable job of deciding how to respond to threats quickly and in a manner that protects students’ safety without causing unnecessary alarm. But that’s a challenge that becomes more difficult as the threats pile up and as freezing temperatures make it impossible for children to wait outside while police search their schools.

“The conditions we face right now, weatherwise, only complicate it that much more,” said Jerry Wilson, superintendent of Worcester County schools on Maryland’s Eastern Shore.

Wilson decided to dismiss all of his schools early on Friday after a pair of threats were called in to the district’s central office and to Stephen Decatur High School.

The high school was evacuated again on Tuesday after the main office got another call saying there was a bomb in the building. By 11:30 a.m., the building had been searched and cleared, and students were on their way back to class.

Parents have been supportive but are concerned, Wilson said. They want to know what schools are doing to handle children’s fear, and what the consequences are for keeping kids home.

Caryn Abbott, the parent of a two Worcester County students, said she felt helpless when she received a text message from her eighth-grade daughter when school was evacuated on Friday.

“I could just hear through her text how frantic she was feeling, and scared,” Abbott said.

Wilson has worked as a superintendent for 23 years, including in Oregon, Wyoming and Colorado. He said that while it’s not unusual for individual schools to receive bomb threats, he hasn’t seen so many threats directed to so many school districts at the same time. “It’s weird,” he said, and it’s frustrating for students who want to be in class.

“They have finals coming up,” he said. The threats are “compromising what we’re able to accomplish in a school environment.”

As educators who aren’t experts in threat assessment, school leaders often lean on law enforcement officials for help gauging threats and deciding how best to respond. In Maryland’s Wicomico County, a bomb threat prompted officials to evacuate a high school Friday. But when a middle school was threatened Tuesday, officials kept it open while police investigated with a bomb-sniffing dog.

“It’s so easy to second-guess our collective decision regarding school evacuations when you aren’t privy to preliminary information,” Lewis, the Wicomico sheriff, wrote in a statement Tuesday. “Decisions to evacuate our schools are not taken lightly.”

In Taunton, Mass., Superintendent Julie Hackett decided to keep schools open after her central office received a voicemail message late Monday night — one of a spate of robocalls that threatened schools in more than a half-dozen towns across the state.

The comments were “non-specific in nature and, therefore, did not warrant an evacuation,” Hackett wrote in an emailed statement. But, she added, police had stepped up their presence at each of the district’s 13 schools.

Schools in the Washington region have been challenged with how to deal with similar threats in recent weeks. Officials in Prince William County evacuated an elementary school after a robo-call threat last week. And since the start of school this year, police in Prince William County have investigated five threatening messages that were graffitied on to school walls at three high schools.

The messages prompted police to increase security at Woodbridge High, Gar-field Senior High and Forest Park High. The messages “do not appear to pose a credible threat,” police said on their Facebook page. “It appears these threats have been done to disrupt school functions and garner attention.”

Moriah Balingit contributed to this report.