Fox News on Sunday excited fringe conservatives on Sunday by citing a single source who claimed that Hillary Clinton had a "medical episode" during the Sept. 11th memorial event in New York City.

While attending a memorial at Ground Zero for the survivors of the Sept. 11 attacks, Fox News correspondent Rick Leventhal tweeted that Clinton left the event because of a "medical episode," citing a single anonymous law enforcement source.

Levanthal later revealed during a broadcast on Fox News that his single source had been 15 feet away from Clinton as she was being escorted to her motorcade.

The report was seized on by alt-right Trump supporters like Breitbart as evidence of the conspiracy theory that Clinton is not fit for office because she suffers from an illness.

But other news outlets waited until additional sources and a response from the Clinton campaign before reporting the incident.

A spokesperson for the Clinton campaign later told reporters that the candidate left because she felt "overheated" and was resting at her daughter's apartment, but was not taken to a hospital as some reports had said.

Baltimore Sun media critic David Zurawik told CNN that Fox News' reporting on the so-called "medical episode" had been irresponsible.

"When I found out about this and it was only Fox reporting it, and it looked like it was one source then it was a little fuzzy about the second source, but I thought, 'Wow, they better be right about this. If' they're wrong this is awful," Zurawik explained.

"On something like this, you wait until you have at least two sources you're comfortable with," he said, adding that CNN had done "textbook" reporting by waiting to find out the facts.

"I was surprised to see anyone reporting this without confirmation."

Minutes later, Clinton emerged from her daughter's apartment, smiling and waving to supporters.

UPDATE: Here's transcript of the Reliable Sources segment above:

BRIAN STELTER: Jeff, thank you very much. Appreciate it. We will stay close to you and check back in. Let me turn back to the panel. David, full disclosure, this happened earlier in the morning on Twitter, a fox reporter reported we have been waiting for confirmation, reaching out to sources in the meantime. That's how it should work in the news business z. The media and political implications of this are. How a story takes root online, how rumors spread before facts catch up. >>

DAVID ZURAWIK: Let me tell you what I thought in my heart of hearts sitting in the green room when I found out about this and it was only Fox reporting it. It looked like it was one source, then fuzzy about the second source. I thought, “Wow. They better be right about this, because if they are not, the possible implications of what they are reporting, if they are wrong, this is awful.” My feeling was, on something like this, Brian, you wait until you have at least two sources you are comfortable with. This is not something you go out there -- it was also reported with details, alleged details I should say about the physical appearance of the candidate and as she got to the van. I don't want to repeat those because I don't know they are confirmed and I never would.

STELTER: Let me give you an example of that. Full disclosure to our audience, people are saying she is at the hospital! She is not. They are outside the apartment building where Hillary Clinton is with her daughter. It's a sample of misinformation that could happen.

ZURAWIK: This is textbook, the careful way it was reported here. Only what they confirmed. If it's worse than this, there's time for it. Wait until it's confirmed with something that is this, possibly, volatile in terms of this election, this close to the election. You go with extreme caution. I was surprised to see anybody reporting it without confirmation. Not online and social media, but elsewhere.

STELTER: Sitting here thinking about the implications of this, I would be worried anytime a candidate had to leave an event like a 9/11 anniversary ceremony because they weren't feeling well. The report is she is feeling better now. What do you think the ramifications of this could be for Hillary Clinton?

KATRINA VAN DEN HEUVEL: Thoughts and sympathy goes to Hillary Clinton. All presidential campaigns have been brutal. This is a scorched earth one. What we are going to witness is further debasement of the tweet. Donald Trump is probably writing his tweet know. The Trumpism system, he has used this to -- for many reasons. It's a proxy issue for the Trump campaign. We are going to see a political debate in this campaign and we, you know, Donald Trump has, with his normalized, hate and fed into American's grievances. Hillary Clinton's second part of her speech was never reported. It's never good to go after voters. Focus on Trump. There's no question Trump and Republicans played dog whistle to prey on the grievances. I think we need to focus on that, with all due respect to secretary Clinton. Health, let's listen to her doctor, not Trump's tweets.

STELTER: He hasn't weighed in yet. Maybe he will. Go ahead.

ZURAWIK: Factually, Katrina said the second half of her speech was never reported. It was in full in the New York Times.

VAN DEN HEUVEL: Not sufficiently.

ZURAWIK: It was in New York Times.

STELTER: We have played the full soundbyte here.

VAN DEN HEUVEL: It wasn't never reported. It wasn't given enough attention, David. Let me clarify.

STELTER: Tara, one more thought on the issue about this health concern and her leaving this event early. There's a difference between conspiracy theories. Legitimate concerns and questions about what is going on with her health. What do you expect as a result of this story?

TARA SETMAYER: It's no accident Donald Trump hired Stephen Bannon who traffics in these conspiracy theories on a regular basis for a reason. They are using these kinds of issues to gin up all kind of feelings. It really takes away the focus from what matters for the presidency. We look at, we reflect on what today is 9/11. Happened that day and the way President Bush handled that with grace and resilience and resolve. It makes us, as Americans, step back and look at the race and the candidates and have to decide, who do you think would be able to step up to the plate and be an honorable representation of this country in a time of crisis? Foreign policies, really, where the President of the United States had the most influence. I think that all the other distractions take away from the fact we have Donald Trump who is grossly inadequate in that area and Hillary Clinton who has inadequacies in a different way. The American people need to see through a lot of the muddy, very personal attacks. It's such a Jerry Springer type of issues that really don't matter to the American people. I think that we need to stay focused on what the office of the Presidency represents and especially on days like today. I hope we don't see a National Inquirer response from Donald Trump. He needs to remember he is running to represent this country.

STELTER: I was going to accuse all of you of pivoting away from my question but it's probably the right response. If we have more reporting and less speculating we will be better off. Stick around here. Just a reset for the audience at home, Hillary Clinton leaving the 9/11 ceremony early this morning said she was not feeling well. She is now better, she was at her daughter Chelsea's apartment in Manhattan. They are saying she is now feeling better.