Here is a good effort from Breitbart to document there being reports of Muslims in New Jersey celebrating the destruction of the World Trade Center on 9/11, backing up the gist of what Trump said.
Anyone who has access to Lexis-Nexis should search it to find reports that are not going to be on the internet -- small local newspapers, for instance. I don't have access at the moment, but most public libraries and research libraries do. If you're a student, you definitely have access. I might do it when I have the time, but others should start in the meantime.
You want to search all newspapers, not just the major ones. Restrict the date to 9/11/2001 through... I don't know, the end of the month, or year? Then search for terms like "World Trade Center" and "Muslims" and "Jersey". If that open-ended search gives too many results to skim, narrow it down by adding "celebrating" or "cheering" or "dancing" or etc.
With all the examples that are turning up from people scouring the internet, there are bound to be many more from established newspapers that can only be searched with Lexis-Nexis.
All the Trump haters are trying to bury these reports or spin them so that it wasn't really a big deal -- so it was only dozens or maybe hundreds, but not "thousands and thousands" like Trump said. Across all the various sites where celebrations took place, maybe it was thousands.
But even if not, the real number to pay attention to is the probability that a Muslim in Jersey City (or wherever) celebrated the 9/11 destruction, when they had the opportunity to. We must compare the number of actual Muslim celebrators on 9/11 to the total number who could have done so. This tells us how likely they were to cheer on a massive terrorist attack on their host nation.
The numerator of the fraction is however many celebrators there were. Let's say it was in the hundreds across the entire city, given reports of "tailgate" style parties.
For the denominator, let's take Paterson, NJ, home to the second-largest Muslim population in America, and one of the sites named in the reports of 9/11 celebrators. Wikipedia says its Muslim population is 25,000 to 30,000 -- I'll put it at 20,000 back in 2001, since we're talking before the massive immigration of the 2000s.
Now, not all of them were free to celebrate by dancing and holding a tailgate party -- some were in school, some were too old and frail, and some were toddlers. According to the 2000 Census for New Jersey, people aged 20 to 69 made up 30% of the population, and I'll assume it was similar for the Muslims. This leaves 6,000 Muslims aged 20 to 69.
But remember that the two towers were struck from 8:45 to 9:00 on a Tuesday morning, meaning most of the celebrating would be during work hours. I also doubt that eyewitnesses would have seen very much after it got somewhat dark, so again the celebrations would have taken place during the work day -- and possibly more at night that eyewitnesses could not detect just walking or driving around.
The BLS says the labor force participation rate for 2000 was 67%, and it was probably lower for Muslims in New Jersey. Perhaps 50% to 60%. That leaves 40% to 50% left to celebrate, or between 2,400 and 3000 people.
If there were even 30 Muslims in Paterson celebrating on 9/11, that would be at least 1% of all who could have done so. If the various parties added up to the low hundreds, say 300, that would be a mindblowing rate of 10%. It must have been somewhere in that range.
You may think that a chance of 1 in 100 or 1 in 10 isn't that high -- but we shouldn't take this probability in isolation. We should compare to other Americans, whether founding-stock whites, immigrants, other non-Christian groups (Hindus), etc. As far as I've heard, none of these other groups held outdoor celebrations in the wake of the two towers coming down.
The entire US population in 2000 was 282 million -- so perhaps there were a handful around the country who escaped notice, for a rate of maybe 1 in 10 million or 1 in 100 million? In other words, Muslims in Paterson were "only" about a million times more likely than the rest of us to openly celebrate the destruction of the World Trade Center.
This is all back-of-the-envelope stuff here, so maybe they'll turn out to be "only" 10,000 times more likely than the rest of us.
And 1 in 10 or 1 in 100 is not a "low" rate, for the kind of act we're talking about. Not that they inwardly felt neither grief nor excitement -- but outwardly cheering on the largest terrorist attack on America. Imagine a mosque in Paterson with 100 regular members -- then somewhere between 1 and 10 of these seemingly innocent regular religious attenders from the neighborhood would have revealed themselves to be traitors. That's the thanks we get for letting them into our country.
Even among those who did not celebrate openly, some fraction would have been sympathetic but lacked the nerve or felt enough shame not to dance in the streets during 9/11. It could be up to half or more of the Muslim population there who are bitterly hostile toward their host nation.
And even if they don't go on to blow up another airliner and skyscraper, those aren't the kind of people we want in our communities. They are incapable of integrating, and will corrode whatever sense of trust, norms, and togetherness we may have.
Not that there's a whole lot of trust and solidarity to destroy in New Jersey -- but opening the floodgates of immigration is only going to make it worse (see Robert Putnam's study on diversity weakening trust in communities). And certainly the rest of the country, where there are strong communities, does not need a single Muslim immigrant. And those who are already there may be hostile to their host community, and some may need to be kept an eye on.
All journalists are innumerate, so I don't expect them to understand the need to focus on a probability rather than a count, let alone how to work with fractions and make use of publicly available data. They get paid too much money to use their brain.
But what's really disgusting is how they're trying to whitewash the whole thing from history. Do your part and go search Lexis-Nexis to add to the growing list of reports of Muslims in America (especially New Jersey) celebrating on 9/11.