The pitch of the voice was computer-altered, making the speaker sound a little like Donald Duck, but the message was hardly cartoonish.
"In a short time, a large number of Jews are going to be slaughtered," warned the squeaky-voiced caller to a Jewish community center, according to a recording from last month that was obtained by the Jewish Telegraphic Agency, a news service.
Threatening calls are pouring in to switchboards of Jewish schools and community centers with alarming frequency. Although authorities said no bombs had been located so far, the threats were tying up bomb squads and detectives, terrifying children who have been repeatedly evacuated from their classrooms and prompting some worried parents to withdraw their children.
The latest rash of threats on Monday hit 20 Jewish institutions in Alabama, Delaware, Florida, Indiana, Maryland, Michigan, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island and Virginia. It was the fifth round of similar threats that have come during the last two months, according to the Anti-Defamation League, which has counted 90 bomb threats this year.
The Anti-Defamation League's San Francisco office received a bomb threat Monday afternoon and evacuated its offices without incident, the organization said.
"One threat or evacuation is one too many, and yet we've now seen more than 20 incidents in a single day not just to ADL, but to children's schools and community centers — and more than 90 incidents since the start of this year," said Jonathan A. Greenblatt, the league's chief executive. "The level of threats and incidents is astounding and must not stand. We will do everything in our power to combat this wave of anti-Semitism."
Some Jewish community centers in Northern California received calls several weeks ago, the organization said.
Monday's bomb threats set nerves on edge coming on the heels of two separate incidents in Philadelphia and St. Louis where headstones in Jewish cemeteries were toppled.
Whoever is behind the bomb threats has stumped investigators with computer software that disguises the origins of the call and modifies the voice. Recipients of the threats have sometimes described them as coming from robocalls, although investigators believe that live callers are making repeated rounds of calls.
"They were all eerily similar in their methodology…. They are leveraging technology, which makes the investigation difficult," said Paul Goldenberg, who used to head a hate-crimes office for the state of New Jersey and is now in charge of investigating anti-Semitic threats for Jewish organizations.
In the past, Goldenberg said, anti-Semitic incidents would increase in reaction to events in the Middle East, such as Israeli incursions in Gaza or Lebanon, but the recent incidents appeared to have no such trigger.
"I am a 20-year veteran of law enforcement, and these are extraordinary times…. I have never seen such an uptick in such a short period of time," Goldenberg said.
Some analysts have tied the upsurge of hate crimes to the polarizing election of Donald Trump as president, but Naomi Adler, head of the Jewish Federation of Greater Philadelphia, said it dated to 2015.
"The rhetoric and disgusting speech began two years before the actual election at the beginning of the election season, but is not the whole explanation," she said. "There is an incredible amount of anxiety and fear in our community."
Over the weekend, more than 100 tombstones were toppled in the Mt. Carmel Cemetery in Philadelphia. The vandalism was discovered Sunday morning by Aaron Mallin, a 36-year-old New Jersey accountant who arrived early to mark the anniversary of his father's death.
"I was shocked by what I saw," Mallin said. "As I walked through the cemetery, I saw more and more gravestones that were toppled down. Some of the older, thinner stones were cracked."
The extent of the damage suggests the attack was not simply the work of teenage pranksters, who frequently are blamed for toppling tombstones.
"Desecration of Jewish cemeteries goes back to the 19th century with the emergence of pogroms," said Michael Meng, an associate professor of history at Clemson University who writes extensively about Jewish history.
A similar attack took place the previous weekend at the Chesed Shel Emeth Society Cemetery in University City, Mo., a suburb of St. Louis.
Trump last week issued a denunciation of anti-Semitism after weeks of requests from Jewish American leaders for him to address their concerns.
White House spokesman Sean Spicer said Monday that Trump "continues to condemn these and any other forms of anti-Semitic and hateful acts in the strongest terms."
In a sharply worded statement to Trump on Monday, Steve Goldstein, executive director of the Anne Frank Center, said:
"It doesn't matter whether you think you are personally responsible for the continued acts of hate against Jews, including today's latest bomb threats. Rightly or wrongly, the most vicious anti-Semites in America are looking at you and your administration as a nationalistic movement granting them permission to attack Jews, Jewish institutions and sacred Jewish sites."
Times staff writer Jaweed Kaleem in Los Angeles contributed to this report.
6:30 p.m.: The story was updated with additional comments from analysts and the White House.
12:40 p.m.: The story was updated with additional information about the bomb threats.