Bomb threats were phoned in to more than a dozen schools in New Jersey, Massachusetts, Delaware and Iowa early on Tuesday, forcing evacuations or lockdowns, law enforcement officials and media reports said.
At least nine schools in New Jersey received phone call threats starting at about 8:50am EST, said Bergen County Sheriff spokesman Anthony Cureton, adding that four had later been given the all clear.
Searches were still underway at five other schools, he said.
The threats - which may have been automated because they were so similar - were dialed in to high schools in Teaneck, Garfield, Tenafly, Clifton, Fair Lawn, Leonia, Bergenfield, Englewood and Hackensack, all in Bergen County, Cureton said.
He said he could not confirm a media report that one of the schools had also been threatened with a mass shooting.
In Massachusetts, Arlington High School just outside Boston was evacuated and students were dismissed for the day early on Tuesday after officials reported a threat of a possible attack using bombs and guns, local police said.
Police said on Twitter they did not regard the attack as credible but that the evacuation was carried out as a precaution. Arlington police said they later determined the threat to be "unfounded."
Nine Boston-area schools were the targets of similar threats on Friday.
Delaware State Police said they were investigating threats made to at least three schools.
The threats made at about 9:30am on Tuesday resulted in elementary schools in Millsboro, Middletown and a high school in Greenwood being evacuated, police said in a statement. Police did not release information about the nature of the threats.
Iowa City West High School, in Iowa City, also received a phoned-in bomb threat on Tuesday, according to local media. The call was received at about 8:40am and forced students and faculty to be transported to an off-site location. Neither the school district nor city police could immediately confirm the reports.