Schools in several communities in the Northeast were evacuated or locked down Tuesday amid threats of violence.
It's unclear if any of the threats are connected. Law enforcement sources have said the FBI is monitoring the threats while local police lead the investigations, according to NBC News and NBC 4 New York.
Three schools in Delaware have been evacuated, including Long Neck Elementary School in Millsboro, Woodbridge High School in Greenwood and Silverlake Elementary School in Middletown, according to state police.
A fourth — Rehoboth Elementary School in Rehoboth Beach — also received a threatening message, according to State Sen. Ernesto Lopez, who called the perpetrator a "coward" who would be "prosecuted to the fullest extent of the law."
Local police departments send K-9s to sweep the school campuses.
The superintendent at Silverlake said in a message to parents the threat was made during a robo-call Tuesday morning and the situation was "under control," NBC 10 Philadelphia reports. Police have not revealed details of the other threats to Delaware schools.
State police said each threat came in the form of a robo-call.
At least one school district in Pennsylvania was evacuated Tuesday.
Officials with the Chichester School District in Delaware County urged parents against driving to school to pick up their children while police investigated, according to NBC 10 Philadelphia.
Meanwhile, at least 10 schools in Bergen County, New Jersey, received bomb threats or mass shooting threats Tuesday, NBC 4 New York reported.
New Jersey officials said schools in Sayreville, Teaneck, Garfield, Tenafly, Clifton, Fair Lawn, Leonia, Bergenfield, Englewood and Hackensack received threats from someone who claimed to have planted bombs or threatened a mass shooting.
Authorities initially said New Milford also received a threat, but the superintendent said that wasn't the case.
At least three schools received threats over voicemail that appear to have been routed through Bakersfield, California, according to the Bergen County sheriff.
A police source familiar with the investigation told NBC 4 New York four of the affected schools had been cleared by law enforcement and the other five are expected to be cleared shortly.
Police in Clifton said there was "no indication of any validity to the threat."
Students were evacuated Tuesday from at least five schools in Massachusetts, including Arlington High School, Groton Middle School, the Florence Roche School in Groton, Fitzgerald Elementary School in Waltham and the Ayer-Shirley Regional High School, where students were moved to a nearby elementary school.
Authorities are also responding to schools in Tewksbury and Newton.
The superintendent of Taunton Public Schools received a threatening call Monday night, prompting police to send officers to the school Tuesday morning. Authorities determined the message was not specific to Taunton and school remained in session.
The FBI is helping police investigate the threats in Massachusetts, but officials told necn they believe the threats are not credible.
It's the second time in as many weeks that threatening messages have targeted a number of schools in Massachusetts and Delaware.
Four schools in Delaware received bomb threats last Monday, according to a news release posted on the state police Facebook page.
Threats also targeted schools in at least nine Massachusetts communities on Friday, prompting several to evacuate. Many were deemed unfounded.
The biggest headline came last month, when education officials in Los Angeles received an email threatening weapons and explosive devices, prompting the city to shutter all 900 of its public schools.
The threat was found to be a hoax, as was a nearly identical threat sent to New York City schools the same day.