Threats made in recent weeks to local schools, the University Medical Center of Princeton at Plainsboro, Quaker Bridge Mall, a private business and residence have local law enforcement scrambling to determine who is behind these pre-recorded messages. While each case so far has been deemed a hoax, police are taking no chances.

“This is an absolutely despicable crime that is targeting the most precious of our society С our children,” said Princeton Police Chief Nick Sutter, on Monday. “It is certainly causing fear among schools and families. We are working with federal and state agencies, and have top experts partnering with us, and we will not stop until the threats stop and these people are brought to justice.”

The threats have increased across New Jersey in recent weeks. “I don’t use this word often, but from my perspective it certainly is an act of terrorism,” Mr. Sutter said. “It causes fear, has economic repercussions, and makes people afraid to go to public places. It’s quite serious in all of its ramifications.”

Last month, John Witherspoon Middle School, Riverside Elementary School, Johnson Park Elementary, and Princeton High School were each the target of threats, known as “swatting” because they draw a heightened response from a SWAT team. After thorough investigations by law enforcement, no suspicious activity was found at any of the schools.

On May 27, the University Medical Center of Princeton at Plainsboro was put in lockdown after an automated phone call to New Jersey State Police said there were gunmen in the hospital and parking lot. A “code silver” was issued and there were rumors that someone had been taken hostage because of the alert, but no suspicious activity was found by state and Plainsboro police.

A day later, shoppers at Quaker Bridge Mall were evacuated for two hours after a call came in from what appeared to be a computer-generated voice. K-9 units from the New Jersey State Police, the Mercer County Sheriff’s Office, and the Princeton Police Department searched but did not find any explosive devices.

Similar hoaxes have taken place in recent years, but the current threats are different. “I’ve been doing a lot of research on this, and it’s been going on for some time,” said Mr. Sutter. “This takes the old-fashioned type of bomb threat that we’ve dealt with forever to a new level. It’s a huge public safety concern. I’ve seen it before, but this is something new.”

The police are working with other agencies to try and teach the public how to best deal with the phoned-in threats. “What we’ve been suggesting to the community, merchants, and the schools is that when a call comes in or is suspected, it’s important to remember specifics,” Mr. Sutter said. “Record the information that is given, the phone number, the information that comes up on the caller ID, and the sound of the voice, and give that information to the police department.”

Some two dozen threats in all have been documented in New Jersey over the past year. Among the targeted locations were schools in Holmdel, Ridgewood, and Farmingdale, as well as the Garden State Mall. The Office of Homeland Security and the Federal Bureau of Investigation have been aiding the New Jersey State Police and municipal police departments such as Princeton in investigating the incidents.

“We know that there are towns nationwide that are getting these, so that’s certainly an avenue we’re examining,” Mr. Sutter. “We’re working with different agencies, comparing all the data, and that’s definitely helpful in several ways. I’m confident that we’ll get to the bottom of it. It’s just really hurtful and has tremendous repercussions for the community.”