EDITORIALNO ONE in Boston will forget the drama of the manhunt for two brothers suspected in the Marathon bombings. Nor will anyone forget the courage of the officers involved, including Sean Collier of the MIT police, who lost his life, and MBTA Transit officer Richard H. Donahue Jr., who was seriously injured. On Thursday night and Friday, residents of Greater Boston experienced the type of siege that most see only in movies. All along, though, the cooperation among authorities, victims, and an intensely engaged public made it clear there was no way for the bombers to escape accountability by remaining undetected. The brothers Tamerlan and Dzhokhar Tsarnaev appeared in surveillance images to have dropped off bomb-laden backpacks near the finish line. Their baseball-capped faces look like those of ordinary young men almost anywhere in the country. But they also fit the profile of a growing danger faced by American communities: disaffected young people, prone to one form of radicalization or another, hoping to act out their frustrations in a blaze of carnage. It’s a threat that Boston and the rest of America will be contending with for the foreseeable future. As global networks, such as the Al Qaeda operation that struck on 9/11, lose their leaders and their maneuvering room, smaller groups and individuals, operating with home-made weapons, will become more prominent. As of Friday evening, authorities were probing the six months Tamerlan Tsarnaev reportedly spent outside the United States in early 2012; investigators are looking for links to overseas terrorists. The brothers are just the kind of young people such networks are pursuing: amateur operatives with “clean hands” — that is, no previous ties to radical groups to put them on watch lists. But the brothers could just as easily have been “self-radicalized,” like the chaplain who perpetuated the shootings at Fort Hood. The Tsarnaevs came to the United States from the former Soviet Central Asia. Their family originally comes from Chechnya, which fought two bloody wars of independence, and where some fighters came to embrace radical Islam. The younger brother, Dzhokhar, made connections with friends and neighbors in Cambridge, where he won a local scholarship, and at UMass Dartmouth, where he studied and played sports. Yet news accounts Friday suggested he readily followed the lead of his brother, a boxer who once declared he had no American friends. Tamerlan, in his frustration, seems to have found solace in the teachings of an extreme Salafist imam, whose speeches are referenced on a YouTube account bearing Tamerlan’s name. This is a threat that can’t be contained through fences or wars. Though all this may feel exotic to many Bostonians — distant and alien — the basic storyline is not. Young people without secure family relationships and communities are prone to radicalism of many varieties. The appeal of a charismatic imam isn’t all that different than a charismatic white supremacist, anti-abortion militant, or animal-rights extremist: All have been known to motivate bombings in the past. The burden of keeping young people from embracing radicalism falls, inevitably, on parents and families, communities, and ultimately law enforcement. The FBI and other agencies closely monitor the Internet activity of extremist groups, and must strive to adapt their intelligence-gathering capacities to the latest ways that young people communicate with each other. Local police must embrace that mission as well. On the home front, parents can monitor their children’s Internet addictions and associations, seeking help when needed. And where there are no family members, others must fill the gap. Ironically, such connections existed for the Tsarnaevs through schools and universities. Both young men availed themselves of Massachusetts public higher education. Cambridge, with its earnest embrace of diversity, was seemingly among the most hospitable of environments for newcomers from overseas, and practicing Muslims. And yet it appears that Dzhokhar and Tamerlan Tsarnaev found radicalism — or it found them. This is a threat that can’t be contained through fences or wars, even though new security efforts should be pursued. Rather, it has to be fought at the human level. The best way to protect communities in Boston and across the nation is by combating foul and extremist ideologies of all stripes, through monitoring, countering with moderate appeals, reaching out to vulnerable young people — and calling the authorities when necessary.