WASHINGTON – By all accounts, the paths traveled by the Tsarnaev brothers in their new American lives had begun to diverge.
Tamerlan, 26, the elder brother, turned more deeply to his Muslim faith as his once-promising boxing prospects faded. Dzhokhar, seven years his junior, won a college scholarship, gained U.S. citizenship and was seen by friends as embracing the opportunities of his new country.
Still, their lives were far more tightly bound than outsiders might have perceived. They shared a dark, secret connection until the moment that the FBI posted their images as the terrorists suspected of making and detonating twin pressure-cooker bombs near the finish line of the Boston Marathon on April 15.
Such sibling plotting — which is particularly difficult for law enforcement personnel to detect — has precedents, said John Horgan, director of the International Center for the Study of Terrorism at Pennsylvania State University. In such instances, siblings aren't inclined to confide in outsiders, making their plans more covert than actions by a loose-lipped "lone wolf" operator or a terrorist cell, he said.
"It's actually far more common than people realize, but it's just one of those things that are not really recognized and talked about," said Horgan, who also is an associate professor of psychology.
Spotting terrorist threats relies heavily on tactics that are more difficult to use with family members, such as recruiting an informant or spotting unusual communications, said two U.S. officials familiar with counterterrorism methods.
There is a considerable record of how family bonds, coupled with a shared sense of grievance, have drawn siblings to terrorist acts in Northern Ireland, the Mideast, and Russia's Chechnya region, where the Tsarnaev brothers have their family roots, Horgan said.
Studies of terrorism show that those three factors — a profound feeling of loss or failure, an ideology that addresses that humiliation and a reinforcing social process — are the common elements motivating individuals to commit a violent act.