BRUSSELS, Belgium — As investigators struggle to uncover the motives behind the Tsarnaevs' alleged deadly attack on Boston, the extended trip of elder brother Tamerlan to Russia's restive North Caucasus region last year may hold the key.
Tamerlan Tsarnaev arrived in Russia in January 2012 and left six months later to return to his home in Boston.
Family members report him visiting his father and other relatives in the Caucasus region of Dagestan and he is thought to have traveled to neighboring Chechnya, the family's ancestral homeland.
Beyond that, reports of his movements are murky.
The surviving accused bomber, Dzhokhar Tsarnaev, has reportedly told investigators he and his brother acted alone in planning the attack and planting the devices that killed three people and injured over 200 close the finish line of the Boston Marathon. Tamerlan is believed to be the plot’s instigator.
However, questions remain about how the disgruntled amateur boxer could have turned into an indiscriminate killer.
It's not clear if his trip brought him into contact with the Al Qaeda-linked insurgents who have been fighting against Russian rule in North Caucasus region for almost two decades, whether he received training from them, or if the brothers' alleged attack was somehow inspired by Tamerlan's experiences in the region.
Relatives say the elder Tsarnaev spent at least a month with his father in Dagestan's seaside capital Makhachkala.
He’s reported to have helped with house repairs, slept late and spent a lot of time praying and studying the Koran. Although some locals say he frequented a radical mosque, family members deny he traveled to the mountainous strongholds of Dagestan's Islamist rebels.
Even in Makhachkala, however, insiders say he would surely have faced frequent exposure to the deadly conflict between the militants and Russian security forces.
"If you go to Makhachkala, you hear people killed every night, there are attacks against the police, or somebody setting off a bomb. I'm talking every day, not once a week or once a month," says Mairbek Vatchagaev, a historian and former official in the Chechen government that tried to break away from Russia in early 1990s.
"The official line from the Kremlin is that everything is fine in the North Caucasus and that tourists should come and visit, but the reality is that there’s a war going on there," Vatchagaev said from his home in Paris. "I don't have the exact figures, but last year alone there were around 700 people killed in this conflict between the Russians and the radical Islamists."
The region's largest Islamist rebel group, the Mujahideen of the Caucasus Emirate, issued astatement denouncing "speculative assumptions" that Tsarnaev may have been associated with it.
"The Caucasian Mujahideen are not fighting against the United States of America. We are fighting against Russia," it said.
However, the group's leader Doku Umarov has previously railed against the United States as an enemy. Preachers spreading the radical forms of Islam, which have taken hold in the region since the brutal wars with Russia in the 1990s, frequently denounce the West as well as Russia.
A July 2011 United Nations document explaining the UN decision to blacklist the Caucasus Emirate as an Al Qaeda associated terrorist organization, cites Umarov calling on his supporters "to engage in war not only against the Russian State but also against all states."
"Once the infidels have been driven out, we must take back all lands that were historically Muslim, and those borders lie beyond the borders of the Caucasus,” Umarov says in a video release cited by the UN.
The Caucasus Emirate claimed responsibility for the January 2011 bombing of Moscow's Domodedovo airport that killed 37 people and the 2010 suicide bombings of the metro system in the Russian capital by two Dagestani women that left 40 dead. Since then, however, the group says Umarov has ordered a halt to strikes against civilians.
Russian security officials cited by local media say they have no records linking Tsarnaev to extremist groups there.
“If this young man had indeed been mixed up in something, and if his activities undermined the country’s security, he would not have left the country,” Magomed Baachilov, spokesman for the Dagestan Security Council, was quoted telling the RIA Novosti agency. National law enforcement officials in Russia have also been cited saying they are unaware of Tsarnaev having any links to the radical groups.
However, the older Tsarnaev was clearly on the Russian authorities' radar screens in 2011, when the Federal Security Service, or FSB, informed the FBI of their worries over his growing support for radical Islam during an early visit home to the Caucasus, where he was seen attending the same radical mosque in Makhachkala.
The FBI interviewed Tamerlan Tsarnaev after the warning, but found no evidence of either domestic or international terrorist activity. The bureau is now facing questions in Congress over whether there were failings in handling the Russian tip-off.
An FBI statement last week said the bureau had requested more specific information from the Russians at the time, but received no reply. Specialists say despite political tensions between Washington and Moscow, cooperation between US and Russian security forces will pick up as the investigation focuses on Tsarnaev's Dagestan connection.
"The Russian side is fairly systematically schizophrenic in the sense that at the top political level, they are very happy to take rather antagonistic positions and non-cooperative positions, but they are also very pragmatic when they need be to cooperative," said Michael Emerson, a former European Union ambassador in Moscow.
However, he told GlobalPost it was unlikely the Boston attacks would lead to any effective Russian action against the Islamist radicals in the Caucasus.
"They are stretched to the maximum and there is no easy variable to turn on a greater intensity of policing in the North Caucasus," said Emerson, now a senior analyst at the Center for European Policy Studies think-tank. "They're desperately stretched and considerably ineffective there as it is, and they can't change that."