“Something has been released over a period of, I would say, a couple of years now,” Mr. Segal said. “Some of it is people literally feeling emboldened by the divisive rhetoric. Other parts of it, frankly, are the ability of people to harass and create fear with much more ease than any other time in human history.”

Jewish organizations have long been aware of security risks. Indeed, the first of 18 best security practices recommended by the Anti-Defamation League is to “make safety and security part of the culture of your institution, involving staff, leadership and constituents.” Another recommended practice is developing a bomb threat response plan.

“Security is part of the culture of the Jewish community in this country and, frankly, around the world,” Mr. Segal said. “You can be prepared and outraged at the same time, and concerned at the same time.”

Still, synagogues, not community centers and cemeteries, have historically been seen as the most threatened. Mark Potok, a senior fellow at the Southern Poverty Law Center, which also tracks anti-Semitic activity, said that he thought that the recent harassment of community centers was driven by the perception that they were so-called soft targets, especially in comparison to houses of worship.

“The kind of thing like the bomb threats strikes me as being trolling on steroids,” Mr. Potok said. “It’s not actually blowing up anything, but it’s causing an incredible amount of havoc, and it’s possible to do anonymously.”

Extremism researchers say that a small group of people could be behind the phone calls and that it is to soon to know whether the vandalism in Missouri is directly connected to the bomb threats.

But Mr. Potok cautioned that the desecration of the cemetery should be seen as alarming.

“My guess is it might be bored and extremely unpleasant teenagers, but it’s not pure happenstance that it’s a Jewish cemetery,” he said. “They may not be big-time, serious anti-Semites who deny the Holocaust and all the rest, but they might be little gangsters who are prodded on by the enormous amount of anti-Semitic propaganda.”