There is no evidence that Nuno F.G. Loureiro, an M.I.T.-affiliated scientist who was shot Monday at his home in Brookline, Mass., was killed in an antisemitic attack. It’s not even clear that he was Jewish.

But in the hours after his death Tuesday morning, a rumor spread that Loureiro was Jewish — and targeted for his pro-Israel politics. In the wake of a mass killing at a Hanukkah celebration in Sydney, Australia, prominent Jewish social media influencers pointed to Loureiro’s death as proof that Jews all over the world were under attack.

The claim appeared to originate from Ira Stoll, the author of a conservative-leaning Substack newsletter called The Editors. In the newsletter and on X, Stoll reported Tuesday that Loureiro was Jewish. On Substack, Stoll attached a screenshot of a Threads post in which a user with that name defended Israel and criticized Hamas.

There was just one problem: The Threads account did not belong to the slain M.I.T. professor. But in an online information ecosystem that rewards virality, paranoia and hot takes — and whose most influential voices are rarely beholden to journalistic ethics — the unverified assertion took hold.

“Loureiro has been reported to be Jewish with strong pro-Israel views,” the pro-Israel account StopAntisemites shared with more than 350,000 followers. Quoting that post, pro-Israel activist Eyal Yakoby wrote to his 250,000 followers on X, “Every Jew must arm themselves.”

Influencers who repeated Stoll’s claim stated it as fact, usually without stating their source of information. If they had, other uses might have seen that Stoll deleted the X post, and edited his Substack article to include a clarification that MIT had clarified the Threads account belonged to a different person.

Instead, the unverified claim spread to other platforms.

“It’s Jew-hunting season,” the pro-Israel food influencer Gabriel Boxer, who goes by Kosher Guru, and the Jewish account Community News told nearly 400,000 Instagram followers in a joint post. Marnie Perlstein, an Australian Jewish influencer, asked in a Reel why the media wasn’t talking about Loureiro’s Jewish heritage.

There was a good reason legacy media that covered Loureiro’s death, among them the Associated Press and The New York Times, did not report that Loureiro was Jewish: It’s not yet clear whether he was. Indeed, some evidence suggests he wasn’t.

At around the same time as Yakoby’s post, a man named Joah Santos tried to shoot down the rumor, saying Loureiro, a friend of his, was not Jewish and would have never spoken about Israel or Gaza. (The Forward has reached out to Santos.)

StopAntisemites’ post had been reposted nearly 2,500 times and received nearly 600,000 views as of this Wednesday evening, and remains visible on X. Santos’ opposing claim, meanwhile, has been seen only 150,000 times.

The idea that Loureiro was Jewish eventually found its way into Yeshiva World News and the Jerusalem Post, which called Loureiro “a Jewish and vocal pro-Israel nuclear scientist.”

Authorities have opened a homicide investigation into Loureiro’s death; no suspects or possible motives have been disclosed. Funeral details have not been announced.

It’s possible that Loureiro was Jewish — neither the university that employed him nor his family has stated otherwise. But no one has been able to say definitively that he was.

The MIT media relations team told the Forward it could not comment on a staff member’s ethnicity or religion. MIT Hillel did not respond to a voicemail left Wednesday evening.

Bruno Cappi, who described himself as a close friend of Loureiro’s in the MIT physics department, said in an interview that he had worked with the professor since 2016 and that his friend had never mentioned being Jewish during that time. Many of their colleagues in the department were Jewish, Cappi said, with last names typical for Jewish ancestry like Friedman and Rosen; if someone were attacking Jews, why would they go after someone whose Jewish identity was not widely known? “It’s all absurd,” he said.

More than 24 hours after Santos and others tried to correct the record, the articles from the Jerusalem Post and Yeshiva World News remained online. The posts by Yakoby, KosherGuru and Perlstein — none of whom responded to requests for comment prior to publication — also remain up as of this publication. (Some X posts have pending crowd-sourced Community Notes underneath stating he is not Jewish and linking to Santos’ post, but those notes are not currently being shown to all users.)

Additional evidence that Loureiro was pro-Israel was also thin: An X user claimed that a Google Street view image of the professor’s home showed a “Stand With Israel” sign. If the image did depict his building, it had been taken three years earlier; it also showed a multifamily building, and Loureiro — if he did live in the building at the time — did not necessarily live in the unit with that window.

Nevertheless, the claim continued to spread. Around 8 p.m. ET on Tuesday — several hours after the posts from Stoll and StopAntisemites — a Wikipedia article was created about Loureiro, which claimed he was born “to a Sephardic Jewish family.” That claim remained on the article for four hours before a different editor removed it.

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