If you were on Facebook or Twitter this week, you might have seen warnings about an immigration checkpoint in Flatbush, Brooklyn. “Avoid Church Avenue from 35th down to Utica,” one popular post warned. “There’s an immigration checkpoint!!” Similar reports popped up across the country, almost always secondhand, spreading like fire.

For anyone worried about being caught up in President Trump’s recent deportation push, such a message might seem urgent to share. A Facebook search shows more than 100 different instances of the Flatbush warning shared since Thursday, with countless more on other platforms. On Sunday, the warnings became loud enough to trigger an alert on the controversial Twitter-monitoring service Dataminr.

The only problem is, the Flatbush checkpoint isn’t real. None of the posts include photos or firsthand accounts, and no one who’s gone down to the intersection has found anything resembling a checkpoint.

For weeks, the New York Immigration Coalition has been tracking the Flatbush rumor and others like it. So far, none of them have checked out. “The Flatbush one won’t die,” says the NYIC’s Camille Mackler, who visited the location last week to investigate. “I can assure you there was no checkpoint.”

But the checkpoint’s fictional nature hasn’t stopped the rumor from spreading. It’s particularly difficult because of instructions at the bottom of each warning, directing users to copy and paste the message instead of using the native share or retweet buttons. In theory, the purpose is to protect the original witness from retaliation, but copy-pasting also gets around platforms’ tendency to collapse different instances of the same post or link. That lets the same message travel farther and faster, and makes it harder to squash.

In this case, it also makes it hard to find out when a given checkpoint was supposedly witnessed, making it even harder to debunk or verify. The earliest mention of the Flatbush checkpoint we found was from February 15th, but the warning kept spreading for nearly a week after that. “Facebook’s algorithm doesn’t do any favors here,” says Mackler. “Something starts on Monday and doesn’t pop until Thursday, but they think it’s current.”

The tactic also poses a real problem for Facebook’s nascent efforts to fight fake news. Thus far, the company’s controls have focused around specific links and posts. Once a link is shared a certain number of times, it can be vetted by a third-party fact-checker, which will assess whether the story abides by the basic tenets of journalism. But these reports aren’t journalism, and while the underlying message is certainly trending, it’s been spread out among enough posts that it’s not clear the Flatbush checkpoint would have popped onto Facebook’s radar.

The rumors come after a week of aggressive enforcement actions by Immigrations and Customs Enforcement (ICE), drumming up unprecedented levels of fear in immigrant communities. That enforcement has taken many forms, including workplace raids and an increased willingness to detain bystanders. But ICE has denied using indiscriminate checkpoints — and according to NYIC, there’s reason to take them at their word. Outside of specific border crossings managed by Border Patrol, ICE has to abide by the Fourth Amendment, which means agents need probable cause for each stop. Systematically searching everyone on a given highway or subway car would be difficult to justify. There are ways ICE might circumvent that requirement, particularly if local police are willing to set up the checkpoint, but so far advocacy groups say they haven’t seen those tactics in action.

At the same time, the hoaxes have become so widespread that some groups are preemptively debunking them. Last Monday, the immigrant rights group Make the Road sent out a mass message warning members not to share information about checkpoints they haven’t personally witnessed. “Many false rumors have been spreading in a climate of fear,” the statement reads. “We urge our neighbors to avoid spreading information about ICE activity about which they do not have direct knowledge.” Reached by The Verge about the Flatbush rumor, the group claimed no specific knowledge, but said the vast majority of such rumors are not true.

It’s difficult to say how the Flatbush rumor started — whether it was an innocent mistakes or an outright lie — but the same fears are already being exploited by criminals. On Valentine’s Day, four men posing as immigration agents coerced $250 from a 21-year-old citizen living in Queens, who was afraid for his undocumented parents. New York Attorney General Eric Schneiderman issued a general fraud alert to warn communities about the scam, and ICE has sent out similar warnings about impersonators in the Phoenix area. A spokesperson for ICE said the agency actively investigates any incidents of impersonation, and works with local agencies to keep communities informed of fraud.

But the targets of these scams are unlikely to report the fraud, especially when there really are widespread and seemingly arbitrary ICE detentions taking place. Last week, ICE agents detained an undocumented woman seeking domestic violence protection in court, allegedly tipped off by her abuser. Another raid targeted men as they left a homeless shelter. Those tactics left many immigrants, legal or not, anxious about initiating any contact with the courts or the police.

“It’s just reckless what they’re doing,” says Mackler, referring to ICE’s recent raids. “They’re creating such fear.”