Trump was addressing a campaign-style rally in Florida last Saturday when he launched into a list of places that have been targeted by terrorists.
"You look at what's happening in Germany, you look at what's happening last night in Sweden. Sweden, who would believe this? Sweden. They took in large numbers. They're having problems like they never thought possible," he said.
The comment provoked mockery on social media, as Swedes pointed out that it had been a peaceful Friday in the Nordic nation, using the hashtag #LastNightInSweden to make comedy suggestions about what he may have been referring too – including the possibility that someone had opened a can of fermented herring in public.
On Sunday, the US President finally revealed the source of his comments. And rather than being based on top-secret intelligence that the Swedes weren't privy to, Trump blamed it all on seeing something on Fox news.
My statement as to what's happening in Sweden was in reference to a story that was broadcast on @FoxNews concerning immigrants & Sweden.- Donald J. Trump (@realDonaldTrump) February 19, 2017
The tweet followed a request from Sweden's foreign ministry on Sunday seeking an explanation.
"We have now contacted the American (State Department) to understand and
receive clarity," ministry press officer Catarina Axelsson told AFP.
Sweden's Employment Minister Ylva Johansson, in an interview with Swedish public television, said she also is seeking clarification of Trump's statement.
"We want to know what he means," she said in an interview broadcast on Sunday.
"The US president speaks and the whole world listens. He is speaking about Sweden in a way that we don't understand what he means or refers to -- in connection with acts of terrorism in other countries. It would make sense if we could get an answer to this question."
Users on Twitter, Trump's favorite communication platform, cracked jokes about the apparent miscue using the hashtags #lastnightinSweden and #SwedenIncident.
Former Swedish prime minister Carl Bildt asked: "Sweden? Terror attack? What has he been smoking? Questions abound."
Sweden? Terror attack? What has he been smoking? Questions abound. https://t.co/XWgw8Fz7tj- Carl Bildt (@carlbildt) February 19, 2017
Gunnar Hokmark, a Swedish member of the European Parliament, retweeted a post that said "#lastnightinSweden my son dropped his hotdog in the campfire. So sad!"
Hokmark added his own comment: "How could he know?"
Trump's speech on Friday was aimed at defending his order last month that blocked refugees and travelers from seven majority-Muslim countries from entering the United States.
The order has been suspended by a federal appeals court, and Trump vowed to introduce a new order this week as a means of protecting Americans at home.
He went on to name Brussels, Nice and Paris – European cities that have been struck by deadly terror attacks, insinuating that some sort of incident had happened in Sweden – the latest example of his administration naming a non-existent attack.
Top Trump aides in his month-old administration have faced criticism and ridicule after speaking publicly about massacres that never took place.
White House counselor Kellyanne Conway – who famously coined the term "alternative facts" – referred to a "Bowling Green massacre" during an interview.
She later tweeted that she meant to say "Bowling Green terrorists" – referring to two Iraqi men who were indicted in 2011 for trying to send money and weapons to Al-Qaeda, and using improvised explosive devices against US soldiers in Iraq.
And White House spokesman Sean Spicer made three separate references in one week to an attack in Atlanta.
He later said he meant to say Orlando, the Florida city where an American of Afghan origin gunned down 49 people at a gay nightclub last year.