Antifascist protesters filed into downtown Portland on June 29 and handed out vegan coconut milkshakes during a counterprotest against far-right provocateurs.

After a few short skirmishes involving batons and pepper spray—and after masked protesters punched, kicked and threw milkshakes at conservative videographer and blogger Andy Ngo—the event fizzled. Then, Portland police tweeted.

"Police have received information that some of the milkshakes thrown today during the demonstration contained quick-drying cement," the official tweet says. "We are encouraging anyone hit with a substance today to report it to police."

National media outlets NBC News, CBS News and others promoted the claim that leftist protesters had mixed milkshakes with quick-drying cement before throwing the sludge at their political opponents. Right-wing media spread it even more enthusiastically.

Antifascist organizers have publicly called the allegation "a misinformation campaign blatantly manufactured by the Portland police to disrupt a thriving antifascist movement."

And WW has learned the Portland Police Bureau has little evidence to back up the tweet. In response to WW's inquiries, the bureau initially said one lieutenant had observed a cement-like powder on a single milkshake cup in the field. Also, after police tweeted the allegation, an anonymous tipster emailed a "recipe" for cement milkshakes to the bureau—but its authenticity has not been verified.

On July 1, Robert King, the mayor's senior adviser on public safety and former president of the police union, continued to give oxygen to the cement milkshake story. He said someone who had been hit with a milkshake reported irritation to his skin and eyes "that wouldn't be consistent with ice cream." He released more information about the lieutenant's observations, saying the officer thought the milkshake had a texture and odor similar to cement. Neither the bureau nor the mayor's office is aware of any actual physical evidence that supports the allegation.

No journalists covering the protest in Portland published photos or witness reports of protesters mixing cement into milkshakes.

Throwing milkshakes gained popularity as a protest tactic earlier this year when demonstrators in the U.K. began dumping dairy treats on far-right politicians. The tactic also mocks American white supremacists who use milk as a symbol in memes that promote their fringe beliefs.

"[The tweet] seems to play into the hands of the far-right provocateurs who have been trying to present a resistance to their activities as more dangerous than they are," says Michael German, a former FBI agent and New York University professor who studies law enforcement efforts to combat terrorism.

German says the Police Bureau's tweet should have either made clear the allegation was a rumor being broadcast in an attempt to keep people safe or backed up by clear evidence.

"In a normal situation, it would be a poorly worded and hyperbolic tweet," German says, "but given the context, where there's a history of aggressive action [by police] against those resisting the far right coming into Portland, it seems to suggest there's a continuing problem."