Police are investigating threats against dozens of schools this week in seven U.S. states.

Searches failed to find bombs or other dangers.

Threats were called in to at least 26 schools in New Jersey on Tuesday. Most were to schools in suburbs of New York City, police said.

Tenafly High School students return to their building after being evacuated to the middle school in Tenafly, N.J., Tuesday, Jan. 19, 2016. Authorities say bomb threats were made against schools in New Jersey, but none of the threats are considered valid. (Tariq Zehawi/The Record of Bergen County via AP)

Other threats were called in Tuesday to schools in Maryland, Iowa, Massachusetts, Delaware, Arizona and Florida. Wednesday brought more bomb threats to schools in Delaware and Arizona.

The threats come at a time polling shows Americans worried about terrorist attacks.

New Jersey police officials said voices on the threatening phone calls sounded altered by computers. The calls were traced to Bakersfield, California, police said.

At Clifton High School, a voice mail warned of a bomb. A separate call warned of a "mass shooting" at the school later on Tuesday, said Sgt. Robert Bracken of Clifton Police Department.

The responses to the threats varied, according to news reports. Some schools moved students to other buildings while officials searched the threatened schools. In New Jersey, some students said they waited outside their schools for an hour or more without coats in below freezing temperatures.

Other schools closed for the day. And some kept students "locked down" in their classrooms.

In New Jersey, police used dogs trained to smell explosives.

Police and school officials said Tuesday they could not say if most threats came from a single group or person. They also said they did not know the reason threats were made.

Some police said some calls appear the work of "copy cats." Copy cats are people who repeat an action taken by somebody else.

Last month, threats were made to school administration offices in New York City and Los Angeles. Emails warned of attacks on multiple schools in the two cities. Los Angeles officials decided to close their schools that day, but New York City officials kept schools open.

No bombs were found. Los Angeles and New York are the largest school districts in the United States.

The threats are making some students and parents nervous.

NBC 4 TV News in New York reported some students said they decided not to return to school Tuesday after the "all clear" signal was given because they remained afraid.

Last month, President Barack Obama observed the third anniversary of the mass shooting at Sandy Hook Elementary School in Connecticut. A lone gunman killed his mother and then drove to the school where he killed 20 first graders and six educators.

In Virginia, Prince William County school and police officials met Monday with parents to discuss recent bomb threats at four county high schools.

Insidenova.com reported that officials told parents most people planning school shootings "do not advertise shootings."

"We have to take the threat seriously, but we also have to balance our approach so that we're not scaring the kids," said Police Lieutenant David Burghart, according to Insidenova.com

But a school official told parents they should tell their children to report any threats they hear about from classmates or others.

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Words in This Story

suburb – n. a town or other area where people live in houses near a larger city

alter – v. to change something or someone

voice mail – n. a message left on a telephone answering device

response – n. something that is done as a reaction to something else

vary – v. different responses to the same problem

locked down -- v. to be kept in place

dozens - n. large numbers of people or things

polling - n. an activity in which many people are asked questions in an effort to get information about what most people think about something

according - adv. as stated by or in

coats - n. an outer piece of clothing that keeps a person warm or dry