President Donald Trump said in an interview that a plane loaded with "thugs" was headed for the Republican National Convention the previous week "to do big damage". He provided few details.
The convention concluded last Thursday night with Mr Trump's nomination acceptance speech on the South Lawn of the White House, and he has complained frequently since then about protesters who accosted audience members as they left.
Mr Trump made the claim concerning the plane of "thugs" in an interview on Monday with Fox News host Laura Ingraham, who asked him who he thought was controlling Democratic presidential nominee Joe Biden.
"People you've never heard of. People that are in the dark shadows," Mr Trump responded.
Ms Ingraham said his response sounded like a conspiracy theory.
"No, people you haven't heard of," Mr Trump continued.
"We had somebody get on a plane from a certain city this weekend, and in the plane it was nearly completely loaded with thugs wearing these dark uniforms, black uniforms with gear and this and that. They're on a plane."
Ms Ingraham asked him where they were coming from.
"I'll tell you some time but it's under investigation right now," Mr Trump said. "But 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 tons of people were on the plane to do big damage. They were coming."
Ms Ingraham asked if they were headed for Washington.
"This was all, this was all happening," Mr Trump said.
Ms Ingraham asked where they were getting their money from.
"The money is coming from some very stupid rich people that have no idea that if their thing ever succeeded, which it won't, they will be thrown to the wolves like you've never seen before," Mr Trump said.
Meanwhile, Mr Biden has accused President Trump of stoking violence after a week of deadly unrest catapulted law and order to the top of the political agenda, barely two months before the US election.
The stakes for Mr Biden's speech in Pittsburgh on Monday, in the swing state of Pennsylvania, could not have been higher a day before Mr Trump heads to the city of Kenosha, Wisconsin, to deliver his duelling vision on the upheaval.
Emerging from months of Covid-19 travel restrictions, Mr Biden finds himself suddenly on the defensive, mocked by Mr Trump as weak in the face of events combining leftist anti-racism protests, riots, deadly shootings and right-wing vigilante actions in Kenosha and in Portland, Oregon.
With Mr Trump exalting in the shift of debate from his widely panned handling of the coronavirus pandemic to his favoured theme of crime, Mr Biden risks losing the momentum that has put him ahead in the polls for the Nov 3 vote.
But the 77-year-old Democrat punched back, branding Mr Trump's presidency "a toxic presence in our nation".
"Fires are burning and we've a president who fans the flames rather than fighting the flames," Mr Biden said. "The incumbent president is incapable of telling us the truth, incapable of facing the facts and incapable of healing."
The Democrat sought to turn tables on the Republican in his 22-minute speechdelivered in a mostly empty hall due to coronavirus concernswith the question: "Do you really feel safer under Donald Trump?"
Mr Trump "can't stop the violence, because for years he has fomented it," Mr Biden charged.
BLOOMBERG, AGENCE FRANCE-PRESSE