For more than a week the people of Ferguson welcomed the outside activists who flocked here to show solidarity after police killed an unarmed teenager. By day 11, some residents were having second thoughts.

The intensified mayhem of recent nights, apparently initiated by a small, organised group of agitators, has raised a question mark over the strangers who join the nightly demonstrations.

“Who the hell are you people?” one woman shouted early on Tuesday morning as youths surged towards a phalanx of police, hurling insults, water bottles and, later, molotov cocktails.

With the national guard deployed to quell civil unrest in the US for the first time in over a decade, amid a night of gunshots, teargas, chaos and 31 arrests, it was an urgent question.

Volunteers – locals and outsiders – had for a time kept a fragile peace by linking arms and forcing rowdy youths, many covering their faces with masks and bandannas, back from lines of police with riot gear and armoured vehicles.

“Relax. If we relax, they relax,” said one man with a megaphone. For a time it worked, and it seemed Ferguson could break the cycle of violent confrontation.

Then, about 500 metres down the other end of the avenue, in front of the ruins of a destroyed QuikTrip store that has been a focus of the protests here, a small group of youths tore up street signs and threw bottles at police. Then at least one of them opened fire, triggering a heavy police response of stun grenades and teargas which made the town, yet again, resemble a war zone.

“If you don’t live here you don’t worry about the burning and looting,” said Steve Beale, 27, who works at an electrical goods store. “You don’t worry about stores closing, or losing your job, or walking for miles to buy food. Well, I live here and I do worry.”

Many other residents voiced the same belief that most, though not all, of the looters, shooters and molotov cocktail throwers were outsiders. “I appreciate seeing people coming here to support us,” said T’erell Wilhite, 18, a student who lives close to where a police officer gunned down Michael Brown on 9 August, triggering the protests.

“But some of them are violent. I’m wondering if the negative outweighs the positive. The focus should be on the family and their loss. The rowdiness should stop.”

Facebook Twitter Pinterest Demonstrators stand in the middle of West Florissant Avenue as they react to teargas fired by police. Photograph: Lucas Jackson/Reuters

There is no easy way to confirm the identities of the hard core rioters. They tend to cover their faces and not give interviews while hurling projectiles or fleeing teargas.

But the perception that they are among the visitors has shifted some scrutiny from police to the eclectic clusters of civil rights activists, anarchists, nihilists, socialists, hipsters, artists, Muslims, Christians and Buddhists.

Antonio French, a local alderman who has joined the nightly marches, warned of a new dynamic after Sunday night’s chaos. “There is a small group of people who cannot be defined as protestors/demonstrators. They are more like fighters/rebels/insurgents,” he tweeted.

The concern has given a twist to the criticism that this St Louis suburb, whose population is two-thirds black, is at the mercy of authorities who do not reflect or understand it.

Fifty of the Ferguson police department’s 53 officers are white, including Darren Wilson, who shot Brown six times. Most of the reinforcements from the highway patrol, St Louis County police and National Guard, which arrived on Monday, are also outsiders.

Sonny Dayan, 53, the owner of a store at the heart of the riot zone, blamed police and rioters for his boarded-up windows and absent customers. He suspected local criminals were helping drive the violence. “Guys on drugs, the rapper attitude, nothing political about it for them.” But most rioters, Dayan felt, were outsiders with their own agenda.

Teresa Mithen Danieley, the rector of St John’s episcopal church, said looters and firebombers were distracting from the tragedy which triggered the crisis. “It’s really pathetic. People should not be taking advantage of the violent death of a child. There is a difference between being an activist and being foolish. An activist has discipline, goals and strategy.”

Amy K. Nelson (@AmyKNelson) Amazing scene here at QuickTrip: exiled Tibetan monks here & people are in awe, hugging them, wanting photos. pic.twitter.com/Tk0ZIgsSqD

Some visitors are veteran activists for police reform. Cephus Johnson, the uncle of Oscar Grant, a young black man whose fatal encounter with an Oakland transit police officer was made into a film, Fruitvale Station, said he felt compelled to come. “It happened again, another young black man gunned down.”

Others have generated a colourful and at times carnival-type atmosphere. A man who called himself Django, after the Quentin Tarantino movie, trotted a horse down West Florissant Avenue. There have been Buddhist monks in orange robes, Muslims in purple tunics, evangelical Christians with megaphones quoting Deuteronomy, and Socialist Worker Party activists advocating capitalism’s overthrow.

Some, like a young white Californian who gave his name only as Sam, are drifters with no clear-cut agenda. “I’m an anarchist and I was in the region so I decided to come. What’s happening here is inspirational, people are resisting,” said Sam, who has slept out some nights.

His T-shirt, with “inshallah” written in English and Arabic, drew the attention of a police officer who smiled at demonstrators and said he was praying for them. “You with the Muslim shirt, I’m praying for you too.”

Despite the suspicion over provocateurs many residents continued to express gratitude for their guests, invited or otherwise. “Strength in numbers”, said Gino Addison, 36, whose porch overlooks the site where Brown was killed. “We can’t make this much noise without all this support. We appreciate all this love.”