– 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.

Tamerlan’s disappointment in lost ambitions of boxing for the U.S. Olympic Team may have found relief in ideology, as he was drawn to radical Islamic thought, and he created a “shared reality” with his brother, said Arie Kruglanski, a terrorism researcher and psychology professor at the University of Maryland.

“Together, they formed this bubble, this molecule of extremism, and convinced themselves that the way to go is to basically declare a personal war on America,” he said.

Tamerlan was the extrovert and Dzhokhar the introvert, John Curran of Watertown, Tamerlan’s former boxing coach who hadn’t seen them in a few years, told NBC News.

“The young brother was like a puppy dog following his older brother,” Curran said.

Usha Tummala-Narra, an assistant professor of counseling psychology at Boston College in Chestnut Hill, said it’s possible that the Tsarnaevs may have formed a “cocoon” mentality, similar to Eric Harris and Dylan Klebold, the high school seniors who killed 13 people and injured 24 in the 1999 Columbine massacre in Colorado.

Their mother, Zubeidat Tsarnaeva, told Russian TV network RT that she believes the two were “set up.”