At a rally on Saturday in Florida, making a case for measures like his controversial travel ban, Trump alluded to mass-fatality terrorist attacks in recent months in European cities such as Paris, Brussels and Nice.
"You look at what's happening last night in Sweden — Sweden!" he told the crowd of supporters. "Who would believe this? Sweden!"
But Swedish authorities reported no terror-related incident or other episode involving large-scale violence. A Foreign Ministry spokeswoman, Catarina Axelsson, told the Reuters news agency that the U.S. State Department had been asked for an explanation.
"We are trying to get clarity," she said. The Swedish Embassy in Washington said the same, via Twitter:
But that was just part of a tweet storm built around a Twitter hashtag, #lastnightinsweden and #swedenincident, which yielded postings including an image of a cozy farmhouse set in an idyllic-looking snowscape, an array of riffs on complicated IKEA furniture-assembly instructions and an assortment of tweets pretending to darkly implicate Sweden's perhaps best-known export, the '70s pop sensation ABBA.
Another common Twitter theme was a popular Muppets character the "Swedish chef," sometimes depicted with a cleaver in hand.
Other tweets mused on the night in question: "I guess it was pretty dark," or "It was kinda cold," or "Last night Trump's hair color appeared in the Northern Lights."
Also weighing in was Sweden's official tweeter. Each week, the country's national Twitter account is turned over to a regular citizen, and this week's tweeter-in-chief was a school librarian, Emma Johannson, who found herself responding to a barrage of tweets about the mystery. Among her many responses:
The mystery was cleared up in due course — more or less.
At Mar-a-Largo, the Trump-owned Florida resort where the president spent the weekend, the deputy White House principal spokeswoman, Sarah Huckabee Sanders, said she believed Trump had seen the report, but didn't say whether the Fox segment was Trump's source of information. She told reporters the president was "talking about rising crime and recent incidents in general."
A short time later, Trump tweeted: "My statement as to what's happening in Sweden was in reference to a story that was broadcast on Fox News concerning immigrants & Sweden."
In his campaign-style address to supporters in Melbourne, Fla., the president had specifically referred to Sweden's long-standing tradition of welcoming refugees and migrants.
"They took large numbers, they're having problems like they never thought possible," he said.
Asylum applications in Sweden peaked in 2015, during a surge of migration led by those fleeing countries like Syria and Afghanistan, but the numbers have since been scaled back significantly.
Sweden's overall crime rate remains low, falling since 2005, according to official statistics, but rape rates during the same period have risen, a change partly attributed to changes in the way sexual assault is legally defined.
The bizarre claim capped a week in which some of Trump's senior lieutenants, including Defense Secretary James N. Mattis, Secretary of State Rex Tillerson and Vice President Mike Pence, traveled to Europe to try to soothe European allies rattled by Trump's affinity for Russian President Vladimir Putin and by doubts about the U.S. commitment to the NATO alliance.
Former Swedish Prime Minister Carl Bildt initially took to Twitter to express somewhat lighthearted incredulity that Trump would make an unsupported claim that left many believing the country had suffered a major terrorist strike.
"What has he been smoking?" he tweeted acerbically.
But later, the statesman took a more serious tone, telling Swedish Radio that he considered Trump's seemingly casual disregard for truth to be deeply worrying.
"If we are in a situation where there is tension in the world, we stand between war and peace," he said in the radio interview, according to the Associated Press. "If we then have a president who spreads lots of false rumors, it can be truly dangerous."
Times staff writer Michael Memoli in West Palm Beach, Fla., contributed to this report.