The various false text messages forwarded to many Americans on Sunday and Monday all started a little differently before making the same debunked claim: Martial law is coming.

Martial law is not coming. U.S. politicians sought Monday to remove any doubt and publicly debunked the rumors. But the messages proved hard to stop or even trace, because they were shared in texts, often forwarded by people who meant well.

"Please be advised. Within 48 to 72 hours the President will evoke what is called the Stafford Act. Stock up on whatever you guys need to make sure you have a two week supply of everything. Please forward to your network," one such false text message read.

With social media networks like Facebook and Twitter cracking down on the spread of dangerous misinformation in the face of the pandemic, misleading information and false claims have moved to what experts are calling a literal "game of telephone" in text-messaging apps.

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Some users, even those who have no intention of spreading wrong information, are forwarding along viral rumors and urban legends to push vital information that is frequently untrue.

The text messages have unnerved many Americans who don't know how seriously to take them, as they have often gotten them from friends or relatives. While it's difficult to track how widespread the texts have become, many people on social media have posted screenshots, and the rumors have swirled enough to push authorities to address them.

The National Security Council tweeted late Sunday: "Text message rumors of a national #quarantine are FAKE. There is no national lockdown. @CDCgov has and will continue to post the latest guidance on #COVID19."

On Monday, Sen. Marco Rubio, R-Fla., implored Americans to "please stop spreading stupid rumors about marshall law," saying the rumors were "COMPLETELY FALSE."

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While the federal government does have the power to take some actions for public health reasons, such as testing people entering the country, decisions about enacting quarantines and other local restrictions fall to state governments.

A similar rumor swirled through text messages among people in New York City last week, leading police to formally deny that there were any plans to quarantine the city.

The messages resemble older forms of misinformation — unverifiable rumors sent in letters, then email and now forwarded via group text — that have caught on amid the uncertainty of the coronavirus pandemic.

The supposed "source" of the martial law rumors differs from recipient to recipient. Different versions of the texts seen by NBC News attribute the rumor to "high-ranking military officials," a "close friend ... with incredibly reliable information" and "a source that works for Homeland Security."

The spread of the texts comes as health professionals have warned of an "infodemic" around the coronavirus and as a poll by the Knight Foundation, a nonprofit journalism organization, found that 74 percent of Americans are worried about coronavirus misinformation.

David Iserson, an author who got one of the viral texts that was attributed to a "friend in the mayor's office," said the text panicked its recipients.

"It smelled like bulls---, but it's obviously freaking my friends and loved ones out," he said.

Irene Pasquetto, the chief editor of the Harvard Kennedy School's Misinformation Review, has been tracking this sort of viral spread across messaging apps, mostly in other countries, since before the coronavirus emerged. She said this type of viral rumor spread via text message groups is an example of "hidden virality," in part because the impact of the rumor isn't quantifiable until it has reached a critical mass.

"It's hidden because nobody can see what people are talking about outside one's own immediate network and viral because content on messaging apps has the potential of being very persuasive," Pasquetto said. "It is shared by people we personally know — we have their phone numbers — and we tend to trust."

The term "hidden virality" was originally coined by Rutgers professor Britt Paris in a September 2019 report for the internet research nonprofit Data & Society.

Pasquetto said the way rumors travel in the U.S. is different from the way they travel in other nations, where the messaging app WhatsApp is more prevalent, in part because U.S. text message groups tend to be smaller.

"One difference between the U.S. situation and the way in which WhatsApp is used worldwide might be that on WhatsApp people tend to share rumors in closed groups of about 20, 30 people," Pasquetto said. "Rumors circulating on messaging apps rarely refer to or cite official sources or mainstream media, and they tend to be either satirical or informative but exaggerated and sensational."

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Claire Wardle, a co-founder of the misinformation research organization First Draft at the City University of New York, said part of the problem with "hidden virality" is that there's no sense of how widespread the rumors are, making them harder to rein in.

"It's all about emotion," Wardle said. "When it's 'my friend who works at the Department of Defense, who said there was a secret meeting,' it's the most powerful form of disinfo. Even journalists can't tell you if a secret meeting didn't happen."

"With this, you can't tell if it's malicious actors spreading these rumors or just a manifestation of the game of telephone."

Wardle said such disinformation can spread in text messages in part because social media platforms like Facebook and Twitter have largely cracked down on coronavirus disinformation.

"It's certainly not perfect, but I've seen much braver decision-taking by the platforms. With political speech, there's often two sides, but there's no pushback on this one," Wardle said. "There's no question about harm with this kind of misinformation. It's quite clear, and it shows what can be done when platforms take it seriously."