US President Donald Trump has refused to disavow the QAnon conspiracy theory during a town hall debate in Florida.
Asked by the moderator at the primetime event in Miami whether he would repudiate the bizarre online claims, he said he did not know anything about it.
His Democratic challenger Joe Biden is being quizzed in a simultaneous town hall debate on ABC in Philadelphia.
Opinion polls indicate Mr Biden has a solid lead over Mr Trump, a Republican, with 19 days until election day.
Millions of early ballots have already been cast for the 3 November vote.
The QAnon conspiracy theory holds that Mr Trump is battling a clandestine "deep state" network of political, business, media and entertainment elites, often involving Satanic plots and child trafficking.
When NBC moderator Savannah Guthrie asked Mr Trump on Thursday night whether he would reject the group, he replied: "I know nothing about QAnon."
Ms Guthrie said she had just told him about the group, reports BBC.
The president said that you told me, but what you told me doesn't necessarily make it fact.
I know nothing about it, I do know they are very much against paedophilia, they fight it very hard, he added.
Trump instead targeted antifa, a loose-knit movement of mainly far-left activists blamed by the US Department of Justice for civil disorder in US cities during racial justice protests over the last few months.