Fueling the conservative alarm was the Portland Police Bureau’s own suggestion on Twitter during the height of the protests — later walked back slightly — that anti-fascist protesters had laced their milkshakes with quick-drying cement before lobbing them at the marchers.

With political polarization on the rise, the nation’s fraught conversations over immigration, race and policing are increasingly being held in tense street showdowns punctuated with eggs, bricks and, lately, milkshakes. Portland’s long history of street activism has made the city a central stage for many of those conflicts; this weekend’s protests left the police department facing unusual criticism that it had not only been slow to halt the violence, but with its questionable claim about cement milkshakes, had dramatically fueled the furor.

Mr. Ngo is an independent journalist in the Portland area who works with the online magazine Quillette, a publication which prides itself on taking on “dangerous” ideas — in some cases, writing about genetic notions of race, in others, about the shortfalls of feminism. Mr. Ngo has also written for The Wall Street Journal, including an opinion piece in 2018 highlighting the prominent display of Muslim identity as an example of “failed multiculturalism” in London.

He has a history of battling with anti-fascist groups, with the two sides sharing a mutual antipathy that dates back many months. The conservative journalist has built a prominent presence in part by going into situations where there may be conflict and then publicizing the results.

Before Saturday’s event, anti-fascist organizers had mentioned him by name. Mr. Ngo had written on social media that he was worried about the event.