The motivations of Tamerlan Tsarnaev, 26, killed early Friday, and brother Dzhokhar, 19, who is wounded and in police custody, remain unclear, President Barack Obama said shortly after the second brother was captured on Friday.
"Why did young men who grew up and studied here as part of our communities and our country resort to such violence?" Obama asked.
"How did they plan and carry out these attacks? And did they receive any help? The families of those killed so senselessly deserve answers."
Despite the many unknowns, analysts said the brothers' turn to extremism seemed to have been stoked, not by the years of unrest in their native North Caucasus region of Russia, but on the Internet.
"The Chechnya issue is less relevant than the radicalization process," said Seth Jones, associate director of the International Security and Defense Policy Center at the RAND Corporation, a Washington think tank.
"It seems the issue here is less that they conducted training in camps or not and radicalized in Chechnya, and more that they were involved in a social media radicalization," he said prior to the arrest of Dzhokhar.
The Tsarnaev brothers, ethnic Chechen Muslims, arrived as refugees in Cambridge, near Boston, with their families around a decade ago, according to family members. Dzhokhar would have only been about 10 years old.
Bayram Balci, a Caucasus region specialist at the Carnegie Endowment think tank in Washington, said the uprooting of young people at an early age can make them more vulnerable to being radicalized in later years.
Tamerlan Tsarnaev had a YouTube page in his name, created in August 2012, where he favorited several Islamist videos in a category entitled "terrorism."
It had links to videos of a radical Australian preacher, Feiz Mohammad, and a playlist entitled "terrorists," according to the Washington-based SITE Intelligence Group.
Fiona Hill, a Caucasus specialist at the Brookings Institution think tank, said the conflict in Chechnya is used as a recruiting tool for Al-Qaeda.
"Videos from Chechnya are all over the Internet. They're constantly packaged as part of the Al-Qaeda network recruitment," she said.
Dzhokhar used Twitter and VKontakte -- the Russian equivalent of Facebook -- where his profile identifies "Islam" as his world view, lists information about Chechnya and Islam, and relates jokes about the unfair treatment of Muslims in the Caucasus.
Ben Wittes, an expert on terrorism and national security at Brookings, said the attack in Boston could be construed as domestic but have international resonance.
"The difference is really a question of who, if anyone, the Boston suspects were really in touch with and who, if anyone, may have been directing what they were trying to do and successfully did," Wittes said.
The Fort Hood shootings in 2009, where US Army Major Nidal Malik Hasan is accused of killing 13 soldiers and military support personnel, unfolded in a similar gray area between domestic and international militancy.
Hasan, born in the United States to Palestinian parents, was said to have had contacts with Anwar al-Awlaqi, the American-born radical cleric later killed in a US drone strike in Yemen.
Frank Cilluffo, director of the Homeland Security Policy Institute at George Washington University, said there were many examples of people wanting to fight for Al-Qaeda in their own country.
"Specifically, you've had a number of examples and cases where people who were trying to fight overseas have been turned back around to attack their homeland," he added.
The bombs used in Boston, pressure cookers filled with explosives, reflect the methods advocated by Inspire, the English language magazine published by Al-Qaeda in the Arabian Peninsula, the movement's Yemeni offshoot, which has also urged aspiring jihadists to launch attacks in their own countries.
Brian Jenkins, author of a Rand study on the profile of jihadists in the United States, said 74% of those involved in such plots were American citizens, of which 49% were born here and 29% were naturalized.
"Many of the jihadists identified in the cases discussed here began their journey toward radicalization on the Internet where they found resonance and reinforcement for their frustration and anger," he wrote.