The hoax calls to police departments or suicide hotlines around the country say that a man is considering killing himself and others or that a bomb has been placed in a building.

The address given on the phone belongs to a synagogue that is livestreaming its services. In some cases, the callers watch in real time as police interrupt frightened worshipers. Later, clips of the incidents are posted online.

The incidents are part of a string of 26 “swatting” calls aimed at synagogues in 12 states across the country that the Anti-Defamation League, a Jewish advocacy organization, has tracked for the last month, including at least five in New York City and state. “Swatting” refers to the police SWAT teams that are sometimes summoned in such cases.

In New York, police officers have showed up at synagogues with bomb-sniffing dogs. In North Carolina, worshipers were evacuated. In California, callers said there was a backpack bomb hidden under a bench.

The hoax calls come as the number of antisemitic incidents in the United States last year reached the highest level since 1979, when the Anti-Defamation League began keeping track.

Earlier this month, a federal jury in Pittsburgh voted that a man who killed 11 worshipers in a shooting at a synagogue there in 2018 should receive the death penalty. It is considered to be the deadliest antisemitic attack in the nation’s history,

These incidents are taking place in an atmosphere of heightened concerns over antisemitic incidents targeting Jewish institutions,” said Oren Segal, the director of the Anti-Defamation League’s Center on Extremism.

“What this specific campaign does,” Mr. Segal added, “is meld the technologies that we know amplify antisemitism and enables people to weaponize them through swatting tactics that not only troll the Jewish community, but try to create fear and anxiety without having to leave the comfort of their homes.”

The New York Police Department said that 911 calls had been placed summoning the authorities to three Manhattan synagogues in the last two weeks. The police came each time. Representatives of the synagogues either declined to comment or did not respond to requests for comment.

The police also responded to a swatting call targeting the Anti-Defamation League’s Manhattan office earlier this month, officials there said.

“The N.Y.P.D. is working very closely alongside the A.D.L. and the F.B.I. to fully investigate these incidents as well as carrying out robust outreach with the civic and faith organizations in New York City for ongoing awareness,” a police spokeswoman said in a statement.

Videos posted on the social media platform X, formerly known as Twitter, show police officers at synagogues in Chapel Hill, N.C., and Fremont, Calif., entering the frame and ordering worshipers to evacuate.

The swatting incidents have made it difficult for congregants to worship in peace. At 10:40 a.m. on Saturday, the Fullerton Police Department in Orange County, Calif., received a call from the Los Angeles Police Department, which had been told that a backpack bomb inside a temple would explode in 15 minutes, said Capt. Jon Radus, the operations division commander in Fullerton.

Officers arrived on the scene and evacuated the temple. The Orange County bomb squad conducted a search using bomb-sniffing dogs and no explosives were found.

A recording of the live feed shows a cantor singing when she is interrupted by a rabbi, who places a hand on her arm.

“I am afraid that we need to stop and leave the building right now,” the rabbi says as the cantor removes her headset and worshipers file out of the building.

Captain Radus said local agencies were working with the F.B.I. to determine the source of the call and whether it was connected to the others. Police presence around the synagogue has been increased.

Mr. Segal said the swatting calls posed a different type of threat than more typical antisemitic acts, such as graffiti or slurs.

“It’s the thousands of people that are anonymous, that are watching, that are getting excited by what they’re seeing and that may be animated to take it to the next level,” Mr. Segal said. “This is so dangerous because we don’t know who else is watching and what they might do based on what they’re seeing.”

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