“The layers of this thing are really important, and that’s to understand what the origin of the violence is, what it’s connected to and what it’s not connected to,” said Mayor Mitch Landrieu, who has long made a case for what he calls the city’s cultural economy. “This didn’t have anything to do with second lines, and it didn’t have anything to do with the rich cultural heritage of New Orleans.”

Sunday’s event was a so-called second line parade, the “second line” referring to all those who join in along the route and follow behind the band, making more of a rolling party than the kind of parade one simply watches. They take place nearly every Sunday between September and May, in the poor and working-class back streets of the city.

Such parades are put on by social aid and pleasure clubs, which function as inner-city relief societies, delivering groceries to shut-ins, buying football uniforms and pooling resources to pay for life’s unexpected invoices, like medical emergencies and funeral costs. They also put on parades once a year in the neighborhood they represent, with the brass bands, Technicolor suits and stops at drinking holes along the way. The parades can cost thousands of dollars, even tens of thousands.

For decades, they happened off the bureaucratic radar, without permits and largely unknown to anyone not directly attached to the marchers. For many New Orleanians — black and white — the parades were, and still are, surrounded by an air of menace.