Police in Portland, Ore., on Monday made public an anonymous email they received purporting to contain the recipe for "milkshakes" containing quick-drying cement that were thrown by at least some protesters during dueling rallies over the weekend.
The email was released at the same time that police sought the public's help in identifying three protesters who are wanted on suspicion of assault and robbery for actions during the June 29 rallies.
Police have already made three arrests, and medics treated eight people at the scene, including three police officers.
Members of the so-called Proud Boys, a far right-wing organization, and an anti-fascist group called Rose City Antifa held dueling protests at several locations in downtown Portland. Fights broke out when the groups crossed paths.
Andy Ngo, a writer and photographer for the conservative website Quillette.com, posted on Twitter that he was attacked by anti-fascists and had his camera gear stolen. An article posted Sunday on Quillette.com about the incident said Ngo was hospitalized overnight with a brain hemorrhage after going to the emergency room with bruises and bleeding on his face.
Police tweeted a warning during the rallies that "some of the milkshakes" thrown on Saturday contained quick-drying cement.
A lieutenant in the field observed "a material with a texture and smell that was consistent with concrete," the Portland Police Bureau elaborated on Monday.
The fallout from the protests promised to further erode a rocky relationship between rank-and-file police officers and Portland Mayor Ted Wheeler.
The police union called on Wheeler to crack down on violence in the city by right-wing groups and anti-fascist counter-protesters alike, adding that officers feel their hands are tied.
"It's time for our Mayor to do two things: tell both ANTIFA and Proud Boys that our City will not accept violence in our City and remove the handcuffs from our officers and let them stop the violence through strong and swift enforcement action," said Daryl Turner, the president of the Portland Police Assn. "Enough is enough."
Wheeler was out of the country. He tweeted Monday that Portland "has always been a beacon of free speech" but that those who broke the law would be held accountable.
"We stand against all forms of violence — regardless of someone's political leanings," Wheeler tweeted. "Portland police officers have the unenviable task of keeping the peace. It's a difficult job and hard decisions are made in real-time."
Liberal-leaning Portland is the site of frequent clashes between right-wing groups who come to the city from outside the state to march and the counter-protesters who show up to try to stop them.
The mayor and police have struggled to find a balance between allowing free speech and preventing the violence that often breaks out among demonstrators.