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Misleading text messages claiming that President Trump was going to announce a national quarantine buzzed into cellphones across the country over the weekend, underscoring how rapidly false claims are spreading — and how often it is happening beyond the familiar misinformation vehicles of Facebook and Twitter.

The false texts spread so widely that on Sunday night the White House’s National Security Council, fearing the texts were an attempt to spook the stock market as it opened Monday, decided to directly debunk the misleading claims in a Twitter post: “Text message rumors of a national #quarantine are FAKE. There is no national lockdown.”

But by then the messages already had spread widely, as had similar ones both in the United States and Europe in recent days. Text messages, encrypted communication apps such as WhatsApp and some social media platforms have carried similarly alarming misinformation, much of it with the apparent goal of spurring people to overrun stores to buy basic items ahead of a new wave of government restrictions.

The one claiming that Trump was going to impose a national quarantine included the advice: “Stock up on whatever you guys need to make sure you have a two week supply of everything. Please forward to your network.”

In fact, authorities have warned against aggressive buying that could disrupt supply chains and fuel panic.

Trump addressed the misleading text messages at an afternoon news conference Monday, saying, “It could be that you have some foreign groups that are playing games.”

On the possibility of a national quarantine, Trump said: “We haven’t determined to do that at all. … Hopefully we won’t have to.”

In Europe, one viral video shared on WhatsApp claimed to show shoppers mobbing a store in the Netherlands. A version of the same clip put to music on TikTok, a popular China-based platform for short videos, was viewed more than 4 million times, according to a report last week by disinformation research group Bellingcat, which determined the video was actually from an incident in Germany several years ago.

Independent researchers overall, though, have struggled to track some of the coronavirus misinformation spreading as the fast-moving pandemic and rapidly evolving government actions spur insatiable demand for information — and opportunities to spread falsehoods. Often the messages are traveling from person to person, or through closed groups of people, through email or texts that are not seen by the general public.

Text messages are particularly difficult for independent researchers to trace, especially when messages ― like the Trump quarantine texts ― are delivered as graphical images as opposed to words that computers can more easily analyze. Those pushing misinformation may be changing tactics away from social media to thwart the major platforms’ efforts to catch and block falsehoods.

“As social media companies focus on keywords like ‘covid’ and ‘coronavirus,’ there are pockets of conspiracy theories that are welling up that potentially have some offline impact,” said Joan Donovan, director of the Technology and Social Change Research Project at Harvard’s Shorenstein Center.

Clemson University researcher Darren Linvill said a version of the false Trump quarantine message spread on Twitter throughout Sunday evening and into Monday morning, peaking before midnight East Coast time, but the total numbers were small, with fewer than 200 tweets. He found a few references on other social media.

What most concerned Linvill was that the misleading messages were spreading on cellphone networks, among friends and colleagues, undermining most readily available tracking technologies.

“That is disturbing,” said Linvill, an associate professor of communication. “The fact that this is texts … makes this impossible to track on social media.”

Graham Brookie, the director of the Atlantic Council’s Digital Forensic Research Lab, which tracks disinformation, said misleading text messages can have great effect because they use the same technology that families and friends use to communicate, meaning people are “more likely to believe” what they read.

“It’s a return to an older threat,” Brookie said. “We saw it on email. This is not new. We’re going to need to figure out more mechanisms to learn about it more quickly.”

In contrast to SMS text messages, WhatsApp has developed controls to prevent mass text messages, imposing limits on the number of messages a single person can send at any one time. Forwarded messages and chain messages are labeled, spokesman Carl Woog said.

Twitter said it, too, has developed tools to slow the spread of misinformation, especially when there is a risk of direct harm.

“We will remain vigilant and are committed to collaboration, which is key to protecting the rapidly evolving public conversation around this critical global public health issue,” said Del Harvey, Twitter’s vice president of trust and safety.

A representative for ByteDance, owner of TikTok, did not respond to a message seeking comment.

As the false texts spread about a looming federal quarantine, the NSC began an interagency effort — involving the FBI, intelligence agencies, the Department of Homeland Security and the State Department — to determine who is behind the apparent disinformation campaign.

The texts were often images of purported text messages sent to friends and relatives to make them look authentic, a degree of sophistication that one U.S. official said might show that it is a “broader plot” than something one hacker alone could pull off.

Federal Communications Commission spokesman Will Wiquist said, “Although we have not received any consumer complaints on this yet, we are aware of the issue and are looking into it.”