Trump said in a Monday interview with Fox News that "people that are in the dark shadows" have "control" over his Democratic opponent for the presidency, former Vice President Joe Biden.
"What does that mean?" the right-wing host, Trump ally Laura Ingraham, asked. "That sounds like a conspiracy theory."
Trump went on: "We had somebody get on a plane from a certain city this weekend, and in the plane it was almost completely loaded with thugs wearing these dark uniforms, black uniforms with gear and this and that. They're on a plane." The president did not provide any connection in his story to Biden.
He said he would say "sometime" where exactly the incident took place and that "it's under investigation right now."
"They came from a certain city, and this person was coming to the Republican National Convention, and there were like seven people on the plane like this person, and then a lot of people were on the plane to do big damage," he said.
Asked by Ingraham if they were "planning for Washington," Trump replied, "Yes, this was all -- this is all happening."
But when asked about the story by reporters on Tuesday morning, the president said his story was based on "a firsthand account of a plane going from Washington to wherever" -- changing the direction the plane was flying, compared to what he claimed the day before.
He said he would see whether "the person" who he said told him the information would speak to reporters.
"That person was on a plane, said that there were about six people like that person, more or less, and what happened is the entire plane filled up with the looters, the anarchists, the rioters -- people that obviously were looking for trouble," Trump said before departing on Air Force One en route to Kenosha, Wisconsin.
It was the latest in a long list of conspiracy theories Trump has promoted, from the false claim former President Barack Obama was not born in the United States to recently accepting the support of those who believe in the "QAnon" theory that a global cabal of pedophiles are fighting against Trump.
The White House on Tuesday would not provide any specific information about the president's story.
In response to Trump's allegation that "people in the dark shadows" control Biden, for which he did not give evidence, a spokesman for the former vice president's campaign, Andrew Bates, criticized Trump more broadly.
“Over 180,000 Americans have lost their lives to the pandemic on account of Donald Trump’s devastating mismanagement," Bates said. "He crashed the strong economy he inherited from the Obama-Biden Administration. And the President of the United States is bizarrely highlighting the unrest and division he has stoked, refusing to condemn violence committed by his own supporters. Donald Trump's incomprehensible case for doing even more damage in a second term makes less and less sense every single day."
Later Tuesday, Biden himself responded in an interview with Raleigh, North Carolina, ABC affiliate WTVD. “He just continually lies. He flat out lies about virtually everything. Dark forces behind me? As I said yesterday, do I look like the guy who is in fact gonna -- this socialist left-wing guy who is going to bring hatred to -- I mean come on, the dark forces?” Biden said.
White House deputy press secretary Sarah Matthews said in a written statement that the president was "referring to questions that need answers such as who may be funding travel and lodging for organized rioters."
"An investigation is underway to determine who is funding these organized riots happening across the country," Matthews said.
The White House pointed to an interview acting Homeland Security Secretary Chad Wolf gave last night to Fox News' Tucker Carlson, another far-right television host, in which Wolf said "the Department of Justice is … targeting and investigating the head of these organizations, the individuals that are paying for these individuals to move across the country."
"What we know, Tucker, is that we have seen groups and individuals move from Portland to other parts of the country," Wolf added, pointing to the arrests in Kenosha, Wisconsin, of people from out of state.
"We know they're moving around," Wolf said. "We've seen them in D.C., in Sacramento, and elsewhere. They're organized. We've seen similar tactics being used from Portland and other cities across the country as well. So, we know that there's organization, I know that the Department of Justice is also looking at that as well."
Wolf was responding to a question from Carlson in which the Fox host asked why leaders of "antifa" and Black Lives Matter movements have not been arrested.
A Justice Department spokesperson, Kerri Kupec, said Tuesday that the department continues to investigate "coordinated, criminal activity -- not First Amendment activity" related to continued riots and unrest in cities around the U.S. in recent weeks.
Kupec declined to comment on the story Trump told during his interview with Fox's Ingraham.
One person was killed Saturday night as protests in Portland entered their fourth month.
The deadly shooting occurred as dueling demonstrations roiled downtown Portland.
A car rally in support of President Trump had gathered earlier in the evening in a massive procession that was ultimately met by counterprotesters supporting the Black Lives Matter movement. Portland ABC affiliate KATU reported seeing clashes between the two groups on the streets.
Investigators have not said whether the shooting was connected to the demonstrations or clashes.
According to a separate press release from the Portland Police Department, there were "hundreds of vehicles" that participated in Saturday's rally and a "significant number" drove into downtown as part of the procession. People in the cars "periodically exchanged words" with pedestrians and, at times, fights broke out. There were also some minor collisions, police said.
ABC News' Alexander Mallin and John Verhovek contributed to this report.