During Sunday night’s presidential debate, a Muslim-American voter asked Donald Trump a pointed question about the anti-Muslim rhetoric that has cast a pall over this election: "There are 3.3 million Muslims in the United States, and I'm one of them … With Islamophobia on the rise, how will you help people like me deal with the consequences of being labeled a threat to the country after the election is over?"

In response, Donald Trump looked her in the eye and repeated the same Islamophobic conspiracy theory he’s been pushing throughout his campaign:

You're right about Islamophobia, and that's a shame. One thing we have to do is we have to make sure that because there is a problem, whether we like it or not — and we could be very politically correct — but whether we like it or not, there is a problem and we have to be sure that Muslims come in and report when they see something going on. When they see hatred going on, they have to report it. In San Bernardino, many people saw the bombs all over the apartment of the two people that killed 14 and wounded many, many people. Horribly wounded. Never be the same. Muslims have to report the problems when they see them. And you know, there's always a reason for everything. If they don't do that, it's a very difficult situation for our country because you look at Orlando and you look at San Bernardino and the World Trade Center. Look at Paris. The horrible — these are radical Islamic terrorists.

Trump is spinning a wild conspiracy theory: the idea that American Muslims secretly know who all the terrorists are. He's painting the entire American Muslim community as a fifth column — a devious enemy secretly undermining the very nation in which they live.

But that’s patently false.

Muslims already report terrorist activity and extremist views to the authorities all the time

Muslim Americans are already actively working to prevent terrorist attacks and counter extremism in their own communities.

According to Gallup, "Since 9/11, the Muslim-American community has helped security and law enforcement officials prevent nearly two of every five al Qaeda terrorist plots threatening the United States." Gallup also found that "tips from the Muslim-American community are the largest single source of initial information to authorities about these few plots."

Trump was wrong in the broad contours of his attack on Muslims, and he was wrong in its particulars. Trump referred to the December 2015 terrorist attack in San Bernardino, California, stating that "many people saw the bombs all over the apartment of the two people that killed 14 and wounded many, many people," insinuating that those people were Muslims who chose not to turn in their fellow Muslim terrorists to the authorities. He’s made similar arguments several times before.

According to FactCheck.org, however, this is false:

One friend of a neighbor said the neighbor noticed a lot of packages arriving at the home of the San Bernardino shooters, Syed Rizwan Farook and his wife, Tashfeen Malik, and that they were doing a lot of work in their garage. The friend said the neighbor did not report it because she did not want to racially profile the couple. And a worker in the neighborhood reported seeing a half dozen well-dressed Middle Eastern men walking from the home to a nearby lunch spot on several occasions, which the man said he found unusual but did not report. In neither case did anyone report that they had seen "bombs all over the floor" of the couple’s home, and failed to report it to authorities.

When Trump has made this false claim in the past, he’s said it was because of "political correctness" that the people didn’t report their observations to the authorities. This time, Trump got even more creative with the lie, implying that it was because they were fellow Muslim terrorist sympathizers.

So, just to recap: Asked about the problem of rising anti-Muslim sentiment during this election, Trump responded by accusing Muslims of being terrorist sympathizers.

Then he went further. Just minutes later, Trump implied that many of the Syrian refugees coming to the United States were actually terrorists, saying they are "going to be the great Trojan horse of all time."

Again, this is patently false. As Vox’s Dara Lind has explained in detail, Syrian refugees who want to enter the United States already undergo an extremely intensive vetting process that can last as long as two years, during which the refugees must prove that they have never had any involvement with any group the US would consider terrorists.