Mr. Andrews, a former Defense Department contractor who is now a weapons engineer in the St. Louis area, set to work. Under the auspices of a national group called the Oath Keepers, Mr. Andrews accelerated plans to recruit and organize private security details for businesses in Ferguson, which are receiving the services for free. The volunteers, who are sometimes described as a citizen militia — but do not call themselves that — have taken up armed positions on rooftops here on recent nights.

“It’s really a broad group of citizens, and I’m sure their motivations are all different,” said Mr. Andrews, who is in his 50s. “In many of them, there’s probably a sense of patriotism. But I think in most of them, there’s probably something that they probably don’t even recognize: that we have a moral obligation to protect the weakest among us. When we see these violent people, these arsonists and anarchists, attacking, it just pokes at you in a deep place.”

Mr. Andrews declined to say how many people were assisting in the effort, saying only that the number was “more than five, less than 500.” He estimated that men make up about 80 percent of the volunteers. About 80 percent are white, and 10 percent are black.