"I watched in Jersey City, New Jersey, where thousands and thousands of people were cheering as that building was coming down. Thousands of people were cheering," Trump said at a rally on Saturday.
"It was on television. I saw it," he told ABC News' George Stephanopoulos again on Sunday. "There were people that were cheering on the other side of New Jersey where you have large Arab populations. They were cheering as the World Trade Center came down."
While elements of this conspiracy have circulated on anti-Islam blogs in recent years, no footage or credible reports have ever surfaced of any Arab-Americans or Muslims celebrating in New Jersey as the Twin Towers fell.
Ibrahim Hooper, national communications director for the Council on American-Islamic Relations, told BuzzFeed News Trump was “either mistaken or he’s lying.”
“This has been one of these vile memes on the anti-Islam hate sites for some time, but there’s actually no evidence to support it whatsoever,” Hooper said. “And it’d be interesting if he personally saw it. That would be quite interesting since it didn’t happen.”
On Sunday, Jersey City Mayor Steven Fulop, a Democrat, wrote on Twitter that Trump’s claims were “absurd.”
So, what's going on?
Well, we know that footage circulated in the wake of 9/11 that appeared to show Palestinians in East Jerusalem celebrating the attacks. Here's some old NBC footage:
That's a long way from New Jersey, though, and Trump's target here — as in many recent comments — is American Muslims.
Ironically, the roots of this one may be in another racist online meme: The "Dancing Israelis" conspiracy theory.
In a statement to BuzzFeed News, the Anti-Defamation League, a Jewish group that works to counter anti-Semitism, described Trump's comments as "a variation of the anti-Semitic myth that a group of Israelis were seen celebrating as the Twin Towers fell."
This myth has long circulated, particularly in the Middle East, as part of a conspiracy theory that blamed Israel for the attacks, which were proudly claimed by Al Qaeda.
A group of five Israeli men were detained in New Jersey after the attacks, after a woman called police to report men sitting on the roof of a white van and filming smoke billowing from the World Trade Center, the details of which can be found in this 20/20 report from 2002. The woman said the men were taking photos of themselves with the scene of destruction in the background.
While questions were raised about the men's possible ties to Israeli intelligence, no charges were laid against the men and they eventually returned to Israel.
According to 20/20, one of the men later denied to Israeli media that they had been "celebrating," saying they had simply stopped to record what they were witnessing. "The fact of the matter is we are coming from a country that experiences terror daily. Our purpose was to document the event," he said.
Trump's comments also seem connected to the baseless reports on anti-Islam websites that American Muslims in Paterson, New Jersey, were celebrating on the day of the attacks.
A 2013 story from the anti-Islam website Bare Naked Islam described Paterson as "the city whose large Palestinian population celebrated the attacks on 9/11," but provided no evidence for this claim.
In fact, in 2007 Reuters debunked this myth in a piece on Muslims in Paterson, who the news agency said were "sad" after the attacks:
Paterson was shaken by the September 11 attacks. On that day, a report circulated on some radio stations and Internet sites that Muslims in Paterson had demonstrated in celebration.
Paterson officials promptly issued a statement denying the report, and Muslim leaders insist it was pure fabrication.
... "The Arabs here were sad after 9/11. Believe me, nobody in this community supports bin Laden. He is a criminal. He makes our life difficult here," said Walid Rabah, chief editor of Paterson's Arab Voice newspaper.
And while Trump says he saw this happen, he sure didn't talk about it at the time. Speaking to German media from near Ground Zero two days after the attacks, Trump made no mention of ever seeing any Arabs cheering in New Jersey.
Nor did he bring up the claims in a 2010 discussion with CNN's Larry King in which they reminisced about watching television on the day of the attacks.
Hope Hicks, a spokesperson for the Trump campaign, did not immediately return a request for more information on the basis for the billionaire's claims, but both NPR and the New York Times could find no credible reports of any Muslims celebrating in either Paterson or Jersey City in the wake of the attacks.
Additionally, the independent fact-checking website PolitiFact rated Trump's claim "pants on fire." The website unearthed a Sept. 17, 2001 Associated Press article which referred to "rumours of rooftop celebrations" in Jersey City but described them as "unfounded." Buried within a Washington Post story from a day later was the claim that "law enforcement authorities detained and questioned a number of people who were allegedly seen celebrating the attacks and holding tailgate-style parties on rooftops while they watched the devastation on the other side of the river." However, the newspaper provided no source for this information.
"There’s no conclusive evidence that New Jersey residents celebrated the attacks, and there’s no evidence whatsoever of any demonstrations where 'thousands and thousands of people' cheered," PolitiFact concluded.
Andrew Kaczynski contributed to this report.