Bomb and mass shooting threats were made against more than two dozen schools in New Jersey on Tuesday, along with schools in at least six other states, forcing evacuations and lockdowns that affected thousands of students.

They were the latest in a string of anonymous threats of violence, all apparent hoaxes, made against U.S. schools in recent weeks.

As least 26 schools in New Jersey received the threats by phone starting at about 8:50 a.m. EST, said Bergen County Sheriff Michael Saudino, adding that the schools were all subsequently declared safe.

"When we catch the people doing this, an example is going to be made," Saudino said.

Made in a robotic voice, the threats appeared to come from computer-generated phone numbers that could be traced back to a location in Bakersfield, California, Saudino said.

Bergen County prosecutor Gurbir Grewal said his office felt confident it would help catch the culprits.

"There are digital fingerprints and we will follow up on each and every lead," Grewal said.

The threats, which may have been automated because they were so similar, were received by high schools in Teaneck, Garfield, Tenafly, Clifton, Fair Lawn, Leonia, Bergenfield, Englewood and Hackensack, police said.

Some of the New Jersey schools were threatened with bombs and others with mass shootings, police said.

In Massachusetts, schools in at least 15 communities received threats, state police said. They offered no details but said no hazardous materials or other credible threats were found after searches of the schools.

Nine Boston-area schools were the targets of similar threats on Friday.

Delaware State Police said they were also investigating threats, made by phone in a robotic or computer generated-style voice, to at least three schools.

The menacing calls were made at about 9:30 a.m. on Tuesday and forced the evacuation of elementary schools in Millsboro, Middletown and a high school in Greenwood, police said in a statement. Police did not release information about the nature of the threats.

Threats were also made to schools in communities including Iowa City, Iowa, Portsmouth, New Hampshire, Mesa, Arizona and North Miami, Florida, according to statements from local law enforcement agencies.