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 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.
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.
Family members, as well as those in tight-knit political and religious groups, can avoid arousing suspicion because they're constantly communicating and visiting one another, said the two officials who asked not to be identified because they have access to classified material.
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 in a telephone interview.
There's even a case of twins, the Patterson brothers, convicted bomb makers in the radical Real IRA faction in Northern Ireland, who were sentenced to long prison terms in 2001 after being linked to more than 41 bombings.
Still, how the Tsarnaevs morphed from sibling immigrants to alleged terrorist killers is a question many will be seeking to answer in coming days and weeks. In dozens of television interviews, former friends and acquaintances expressed shock that the two young men with whom they socialized, played video games and studied would commit such a heinous act.
"The real question here is, how do you tell when someone gets radicalized?" Richard Clark, former U.S. presidential terrorism adviser, said on ABC today.
"They're normal, they're happy kids in Cambridge and then something happens, a switch is flipped."
When it is "self-radicalization online" it is tough got the FIB or Homeland Security to trace, Clark said.
President Barack Obama promised that authorities will determine why such an attack happened.
"The families of those killed so senselessly deserve answers," he said in the White House press briefing room after the manhunt ended. "The wounded, some of whom now have to learn how to stand and walk and live again, deserve answers."
As Tamerlan was killed trying to escape the manhunt April 19, the focus is on Dzhokhar, who is being treated at Beth Israel Deaconess Medical Center in Boston for gunshot wounds.
"We're very fortunate that we have one of them alive," Eamonn Gearon, a professorial lecturer at Johns Hopkins University's School of Advanced International Studies in Washington and an expert on the Middle East, said in a phone interview yesterday.
The FBI's High-Value Detainee Interrogation Group, a team designed to question detained individuals who may possess information regarding terrorist attacks, is in Boston, according to officials. Federal charges will be filed against Tsarnaev "in the coming days," Christina Sterling, a spokeswoman for Boston U.S. Attorney Carmen Ortiz, said.
For now, said Horgan, "we are faced with some very, very thin facts with respect to motivation." Multiple pathways lead to terrorism, according to Clark McCauley, co-director of the Solomon Asch Center for the Study of Ethnopolitical Conflict at Bryn Mawr College in Bryn Mawr, Pennsylvania. Some people get involved "for thrill and status, some for love, some for connection and comradeship," he wrote in a 2010 paper examining radicalization factors.
"Personal and group grievance can move individuals toward violence, with ideology serving only to rationalize the violence," he wrote.
Arie Kruglanski, a terrorism researcher at the University of Maryland at College Park, said the evidence so far points to a deeply discouraged Tamerlan, who at one point had ambitions to box for the U.S. Olympic Team.
"He wanted to be somebody important," Kruglanski, a senior researcher at the federally funded National Consortium for the Study of Terrorism and Responses to Terrorism, said in a telephone interview.
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 found relief in ideology, as he was drawn to radical Islamic thought, and he created a "shared reality" with his brother, said Kruglanski, who is a professor in the Psychology Department 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.
The brothers, who lived in Cambridge, came from the former Soviet Union to Massachusetts -- a young Dzhokhar first followed by his brother and two sisters.
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.
Horgan said it is "very plausible" that Tamerlan may have drawn Dzhokhar to a dark view of life after trying unsuccessfully to assimilate into American culture.
Still, he said, "in all the cases we've looked at involving pairs of brothers or pairs of sisters, it's not necessarily the sort of master-apprentice relationship that people think it is."
Usha Tummala-Narra, an assistant professor of counseling psychology at Boston College in Chestnut Hill, Mass., said "sometimes a person can look like they're blending into mainstream culture, and dress like they have, and go to parties and the rest of it, but they may feel like they never belonged."
Brothers like 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 High School massacre in Colorado, said Tummala-Narra, who hasn't met or spoken with either of the brothers.
Their mother, Zubeidat Tsarnaeva, told Russian TV network RT that she believes the two were "set up."
"My youngest son has been in America since he was 8," she said. "We never talked about terrorism at home. My eldest son became interested in religion five years ago. He began to follow rules of Islam, but he never said he plans to enter the road to jihad."
A YouTube page in Tamerlan's name included videos by radical Australian cleric Feiz Mohammad. A profile attributed to Dzhokhar on the Russian social networking site Vkontakte has links to several Chechen-related groups, while a Syrian jihadist video posted April 9 on his page purportedly shows images of atrocities in Syria and ends with the Russian phrase, "Syria calls. We answer."
The FBI is particularly interested in determining whether Tamerlan received training or direction from foreign radicals during a visit last year to see his father in Russia's North Caucasus region, a hotbed of separatist movements, ethnic rivalries and extremist Islamic ideology.
Tamerlan, a legal resident of the U.S., flew to Russia in January 2012 for as long as six months, said two law enforcement officials briefed on his travel. His father, Anzor Tsarnaev, told the New York Times that Tamerlan stayed with him in Dagestan and together they went to Chechnya "to visit relatives."
The Tsarnaev brothers' background may have played a role, said Gearon of Johns Hopkins, noting the legacy of Russia's brutal repression of the Chechen separatist movements over more than two decades. Constant talk of war "can brutalize people," he said.