The recent appearance at a progressive community space in Brooklyn of a conspiracy theorist who blames the 9/11 attacks on “the Zionist war agenda,” is generating debate among lefty Jews over whether some of their own terminology is fueling an increasingly entrenched anti-Semitic fire.

The speaker, Christopher Bollyn, appeared Sept. 7 at the Brooklyn Commons, a Boerum Hill community organizing and event space for left-wing groups. Accompanied by slides, Bollyn spoke for more than two hours, telling his two-dozen or so listeners that the destruction of the World Trade Center was a “false-flag operation” conducted — and later covered up — by the Israeli government with help from American Zionist Jews.

“The Zionist war agenda waged by the U.S. was the primary reason for 9/11,” said Bollyn, whose self-published book on the subject calls the tragedy “a monstrous Jewish-Zionist crime of our time.” The purpose of the plot, he said, was to launch a war on terror, reduce the Mideast to a “patchwork” of “weak and powerless” ethnic states, and, ultimately, to establish “Jewish military and economic hegemony over the entirety of Eretz Yisrael.”

Delivered in a composed, measured tone that a Haaretz reporter compared to that of a “small-market news anchor,” the presentation also managed to link Menachem Begin, Shimon Peres and Benjamin Netanyahu to a coterie of American Jews, including World Trade Center owner Larry Silverstein, Chicago Mayor Rahm Emmanuel, financial executive Maurice Greenberg and Federal Judge Alvin Hellerstein, who presided over the 9/11 lawsuits.

But as smooth as Bollyn appeared, passions were roiling both outside the building, where about 20 protesters gathered, and within the back room, where he made his presentation. Some of the protesters tried to enter the building, but were turned back by a security guard hired for the event. A scuffle broke out, leading to the arrival of police officers. And inside the room, once it became clear that Bollyn’s talk would exceed two hours, leaving little time for questions, one member of the audience rose to express his frustration and was subsequently kicked out.

The talk took place despite an organized campaign asking the venue’s owner, Melissa Ennen, to cancel Bollyn’s appearance. Much of that effort was led by Daniel Sieradski, national organizer of Progressive Jews PAC, who wrote about Bollyn’s scheduled talk on his blog, Jewschool.

Sieradski told The Jewish Week that he was especially disturbed that a speaker like Bollyn would appear at “a progressive venue like the Commons, where progressive groups [are supposed to] feel at home.” That, in itself, creates a special obligation on the venue’s part to bar speakers like Bollyn, he said.

Bollyn, who describes himself as a journalist, has worked for Spotlight and the American Free Press, both white-nationalist publications; has quoted The Protocols of the Elders of Zion, “the most notorious and widely distributed anti-Semitic text of modern times,” according to the U.S. Holocaust Memorial Museum; and has attended Holocaust denial conferences in the United States, according to the Anti-Defamation League. During a 20-minute question-and-answer session at last week’s event, Bollyn told The Jewish Week that he’s been to Auschwitz but hadn’t seen any evidence that gas chambers once existed there.

After getting the background on Bollyn, Sieradski contacted the left-wing organizations housed at the Commons and nine organizations, including the Marxist Education Project, WBAI, Right to the City Alliance and Jacobin Magazine, issued a statement saying they “do not have any say in event booking and management at the Commons but agree that such politics should have no place in leftist spaces.”

Other statements came from Jewish groups advocating an Israeli withdrawal from the West Bank including the New York chapter of Jewish Voice for Peace, Jews for Racial & Economic Justice. One such group, If Not Now, wrote, “Criticizing the policies of the Israeli government — or any government — is not anti-Semitism. Blaming a cabal of malicious Jews for orchestrating the tragic events of 9/11 is.”

Ennen stood firm, writing in a statement that the Commons was not “designed to be a cozy cocoon for intramural debate among leftists.”

In a brief interview with The Jewish Week, she said she doesn’t invite anyone to the Commons or do any programming, but, instead, rents space to individuals or groups who approach her.

“People come to me and pay me money,” she said. “I don’t have time to look into everybody” or vet them. She claimed that she was unfamiliar with Bollyn before the controversy erupted but said she wouldn’t have acted any differently if discovered that a slated speaker had a history of railing against African Americans or Latinos. By way of explanation, she said, she had grown up in North Carolina during the days of Jim Crow and worked as a waitress at a local resort. Ku Klux Klan groups “openly” held conventions there, “and I waited on them,” discovering, in the process, that the experience was educational for her.

“People should be exposed to other people’s hatred and ignorance,” Ennen said. “I really don’t think it’s a good idea to pretend that it’s just not there. I think sunshine is better than darkness.”

Sieradski, though, isn’t buying any of it.

“If she likes exposing hate and ignorance, then why would she … [prevent] activists from entering the space,” he asked. And why, he added, would she allow Bollyn to speak for nearly two hours, leaving little time for questions, when the audience was initially told that his presentation would last an hour, followed by a question-and-answer session?

Sieradski’s research also indicated that Ennen has organized two 9/11 “Truther” conferences, one of which featured Kevin Barrett, a former academic who, like Bollyn, denies the Holocaust and believes that Israel was behind 9/11. For her part, Ennen confirmed in an interview with Haaretz that she was involved in the 9/11 “Truther” movement — the collective term for people who dispute the mainstream account of Sept. 11. But she didn’t blame the Israeli government, saying she didn’t know who carried out the attacks.

Conspiracy theories around 9/11 have become increasingly entrenched in the 15 years since the tragedy, said Marilyn Mayo, a research fellow at the ADL’s Center on Extremism.

The surge in those theories doesn’t surprise Mayo, who believes that “any catastrophic event like 9/11” can be so overwhelming that some people “want easy answers as to why it happened.” But in the age of Trump, whose embrace of conspiracy theories has no parallel among modern presidential candidates, and with the rise of populist, anti-establishment sentiment, observers say that such theories are beginning to move from the fringes to the mainstream.

Bollyn has been on Mayo’s radar for years because the theory he expounds blames Israel and a cabal of Jews and Zionists, she said. But not all 9/11 conspiracy theories are anti-Semitic. Others lay the blame on the U.S. government or the George W. Bush administration.

As word of Bollyn’s upcoming appearance spread, small circles of progressive-Jewish activists began discussing what they could do through Jewschool and through an email listserv, maintained by the Third Narrative, a project of the progressive-Zionist group Ameinu.

One participant in the Third Narrative listserv, David Schraub, published an essay in Tablet magazine on the “five lessons leftists must take” from the “debacle.” The first lesson, a sort of pat on the back for progressive organizations, is that those groups are willing and ready to condemn anti-Semitism among their own, putting to rest a fear among left-wing Jews that they wouldn’t be. Not only did nine of the Commons’ resident organizations condemn their own space for hosting Bollyn, but other progressive venues cancelled Bollyn’s appearances as their owners learned of his views.

But in a challenge to the left, Schraub asserted that there’s “a reason why someone like Bollyn books events at venues like Brooklyn Commons,” while a conservative pundit such as Dinesh D’Souza doesn’t: “He does so because he realizes he can find an agreeable audience in such spaces; that the environment there is receptive to his noxious ideology.”

Progressives need to examine “the elements and attributes” of their own community that offer anti-Semites such fertile ground before they can effectively combat events like Bollyn’s — just as conservatives need to reckon with the bigotry in their own movement rather than pretending that the Trump nomination came out of nowhere.

Similarly, progressives need to counter the Boleyn’s of the world not solely because of their association with far-right and neo-Nazi groups, but also on the basis of their anti-Semitism, said Schraub, a lecturer at the University of California’s Berkeley Law School and a graduate student in political theory at UC Berkeley. In one example of how not to proceed, he cited Jewish Voice for Peace’s attempt to dissociated itself from Alison Weir, a fiercely anti-Zionist activist, by citing the group she was addressing — white supremacists, neo-Nazis and Holocaust deniers — and not her own belief that the medieval blood libel was accurate.

Schraub’s essay caught the attention of Jewish progressives, who are hotly debating many of his points.

While Rebecca Vilkomerson, JVP’s executive director, said she doesn’t believe that 9/11 conspiracy theorists “can be considered part of the left,” she wouldn’t condemn Bollyn’s presentation as anti-Semitic because she wasn’t there to “hear what he said or how he said it.”

But Schraub believes that common sense would dictate otherwise.

“Discrimination is part of the world, the left is part of the world, and so there’s going to be anti-Semitism on the left,” he told The Jewish Week. “Any social group is going to have to do work to overcome the legacy of anti-Semitism — and that doesn’t exclude the left,” he said.

Like other progressives, Schraub is convinced that the progressive movement “has tools to fight discrimination” that many on the right lack. Progressives have worked hard in recent years to understand discrimination, while much of the right lacks that “fine-grained understanding,” he said. As a result, though many conservatives are concerned about anti-Semitism, when it comes to fighting it, “they’re not particularly effective.”

As Schraub sees it, the problem on the left involves not a lack of understanding, but a reluctance to use that knowledge to examine itself. “I think any group, not surprisingly, is reluctant to self-critique,” he said.

Sieradski’s belief is that anti-Semitic conspiracy theories like Bollyn’s originated on the right, where they were given impetus by “crackpots like the [conspiracy theorist radio host] Alex Jones,” but that they’re “seeping into the left.”

Sieradski is also engaged in the type of self-examination that Schraub is urging.

“In our efforts to distance ourselves from Israeli actions,” he said, referring to Jewish progressives, “we sometimes unwittingly provide cover to those who claim that all charges of anti-Semitism are baselessly made to silence critics of Israel. That’s something we need to be cognizant of, and it’s given me cause for reflection.”

In addition to individual reflection, many people are exploring these issues collectively, in diverse forums devoted to dialogue and deliberation.

“Progressives can and should hold conferences on anti-Semitism on the left, as we’ve done twice in my own area,” said Rachel Eryn Kalish, a San Francisco-based expert in mediation and conflict resolution. “Those conferences could cover a wide variety of topics, including those that were raised by the Brooklyn Commons event. The important thing to note is that even the most challenging topics can be addressed without ripping communities apart.”