This is what they had long been fearing. As the threats increased, as the online abuse grew increasingly vicious, as the defacing of synagogues and community centers with swastikas became more commonplace, the possibility of a violent attack loomed over America's Jewish communities.

On Saturday, the worst of those fears was made real as a gunman stormed a Pittsburgh synagogue, killing at least 11 of its members and injuring many more, reportedly shouting "All Jews must die" during his rampage. It is the worst single attack on American Jews in the history of the country. And it is one that many who have been monitoring anti-Semitic activity in the United States have been dreading.

"Unfortunately, in the atmosphere we are in, as shocking as these incidents always are, they are not surprising," said Oren Segal, director of the Anti Defamation League's Center on Extremism. "Anti-Semitism is the lifeblood of extremism, and violence is never that far behind."

In its annual Audit of Anti-Semitic incidents in the United States, the ADL chronicled a 57 percent rise of incidents in 2017 over the previous year. That included everything from bomb threats and assaults to vandalism, desecration of cemeteries and the flooding of college campuses with anti-Semitic posters and graffiti.

Saturday's deadly attack took place against the backdrop of a particularly toxic era in American political and social life. Many Americans believe that the increase in anti-Semitism, xenophobia and racism over the past two years has been stoked by the rhetoric of some of the nation's top leaders, particularly President Donald Trump, whose ongoing rallies are marked by denunciations of immigrants and the deriding of "globalists," which is viewed as a code word for Jews. Most recently, he has declared himself a "nationalist," thrilling some of his followers who identify themselves as white nationalists.

It has been just 14 months since white supremacists protesting the removal of a Confederate statue marched through the University of Virginia campus chanting, "Our blood, our soil!" and "Jews will not replace us!" They were met not with an unconditional rebuke by Trump but a claim by him that there were "very fine people on both sides." On the far right, the president's words were taken as an endorsement of their behavior and their ideas and encouragement to pursue them.

"The response was not at all satisfactory," Segal said. "It's not hard to condemn Nazis or anti-Semites unequivocally. That's the expectation the Jewish community has. That's the expectation all communities have."

Although anti-Semitism is surging, it is not new in the United States. The country's Jewish groups and organizations have long been targets of zealots and bigots. But for much of American history, there have been relatively few large-scale violent attacks. And nothing on the order of Saturday's mass murder. That it took place in such a politically poisoned atmosphere is also significant, observers say.

It's giving permission to people on the margins to act out.

"We have seen acts of violence. What's new is the context of the acts of violence," said Eric Ward, executive director of the Western States Center, an Oregon-based progressive group focused on social and economic justice.

"I have never seen this coupling of political violence with political rhetoric before," said Ward, who has been studying anti-Semitism for the past 30 years. "It has primarily come out of the margins, and what's different about this moment and chilling about this moment is that the rhetoric is now coming out of the mainstream, and it's giving permission to people on the margins to act out."

Deborah Lipstadt, a professor of Jewish History and Holocaust Studies at Emory University, said that in previous decades, such as when she was growing up in the 1950s and 1960s, anti-Semitism was more structural, in the form of discrimination in employment and education. "I knew I had to be twice as good as the non-Jewish kids' to get into college, for example, she said.

Attacks would happen on a personal level, for example. "Kids would be beaten up in the street if you lived in the mixed neighborhood," she said. But Lipstadt said she was taken aback by the scale and horror of the Pittsburgh rampage.

"This is beyond anything we've experienced," she said.

She said it and the recent wave of anti-Semitic incidents over the past two years are the result of "anti-Semitic dog whistles' from leaders - for example, she said, rhetoric painting George Soros as a "21st century Rothschild" - that have emboldened neo-Nazis and other white supremacists intent on committing acts of violence.

The increase in anti-Semitic attacks and harassment online, particularly on popular social media platforms, has been an acute concern in recent years for those monitoring far-right hate groups and white supremacists.

"The rise of the far right in America and Europe is tied to both the spread of anti-Semitic conspiracies about Jewish global domination and increased calls for stronger borders and nationalist policies in majority white countries," said Joan Donovan, media manipulation research lead at Data and Society Research Institute, an independent nonprofit in New York. "Attention to conspiracy theories about Jewish people, especially Soros, has reached new mainstream audiences through Internet memes and right-wing news outlets. This has led to increased harassment and calls for social media companies to ban topics such as Holocaust denial."

Yet more hatred & anti-Semitism from Trump supporters. Swastikas on the playground equipment in Adam Yauch Park in BK Heights. #NeverIsNow pic.twitter.com/Xbcwo4enfF

— Brad Lander (@bradlander) November 18, 2016

Although there are increasing demands for platforms to aggressively monitor and remove such material, not every social site has been vigilant in addressing the problem.

"Such failures to act on the propagation of these conspiracies is dangerous," Donovan said.

Rabbi Jack Moline, president of the Washington, District of Columbia-based Interfaith Alliance, said the current public display of anti-Semitism "is like nothing that I have seen in my lifetime, and I go back to the early "50s."

For Moline, the marches in Charlottesville, Virginia, last year were the first public demonstrations of anti-Semitism he had seen since the late 1970s, when American neo-Nazis fought in court to march in Skokie, Illinois. After the Sept. 11, 2001, terrorist attacks, he remembers synagogues and Jewish centers requiring identification from visitors and erecting barriers to prevent car bombers. Some of the precautions may have seemed excessive. Not anymore.

"I don't know a Jewish institution who hasn't considered the new security requirements of a dangerous world," he said. "I think we have gone from theoretical to practical in a matter of minutes today."

For the ADL's Segal, who has spent 20 years tracking anti-Semitic violence and intimidation, Saturday's deadly news was met with sorrow, but not despair.

"You can't do this work without maintaining a healthy dose of hope that things will get better," he said. "We have to hope that this moment in time will not be remembered solely for the haters and the violence, but for what people did in response. Already we're seeing people coming out in the streets saying that this does not represent us, this does not represent this country. That's a good first step. Now we need our elected officials and community leaders and business leaders to push back against this hate more than ever."

The Washington Post's Michelle Boorstein contributed to this report.