Russian security services monitored Boston bombing suspect Tamerlan Tsarnaev during a five month visit he made to the North Caucasus region of Dagestan in 2011-2012, but found no reason to arrest him, a security source told AFP Monday.
The older of the two Tsarnaev brothers suspected of setting off the Boston marathon bombs, 26-year-old Tamerlan was shot and killed Friday by US police, while his younger brother Dzhokhar was wounded and is in custody.
Tsarnaev travelled to Russia's southern region of Dagestan, where he had previously lived, in late 2011, staying for a total of five months and 13 days, said a security source involved in tracking extremist activity in the region.
"Security services came across Tamerlan four times" in course of his visit, he told AFP.
"They were monitoring another young man, suspected of connections with the Islamist insurgency, and Tamerlan was seen with him."
"But then the young man disappeared from view of the security services, and Tamerlan was no longer seen either," the source added. In the months before he left Dagestan, "nothing was seen for which he could be detained or charged."
US police want to question the surviving younger brother Dzhokhar, a 19-year-old naturalised US citizen, whose injuries make him unable to speak. But reports have suggested that Tamerlan was the driving force in the attack.
No other suspects linked to the brothers, who apparently had more bombs in their arsenal when they were captured, have yet been announced.
An accomplished amateur boxer in Boston, Tamerlan Tsarnaev did not join any boxing club while in Dagestan, where the sport is massively popular, a spokesman for the Russian boxing federation told RIA Novosti news agency.
While in Russia he also applied for a new passport, to replace one he allegedly lost in Boston, a local migration service official told Russian news agencies after finding that the document was still waiting to be picked up.
The Tsarnaev family, which has relatives in both Chechnya and Dagestan, where the brothers' mother is from, was well-liked by neighbours and relatives.
Relatives told AFP in the Dagestan capital of Makhachkala that they can hardly believe that the boys would commit a horrible crime.
"I don't believe that children of such parents could do such a thing," said their elderly neighbour Liza, describing the young men's father Anzor as someone who always helped her out.
The suspects' parents have now fled their apartment to avoid media attention.
Since it was known that Tamerlan came from the United States, Russia asked the FBI for information about him, once at the end of 2011, and once several months after he left, in November 2012, the security source said.
The FBI said Friday that a request from an unidentified foreign government for information about Tamerlan Tsarnaev came as he was "prepared to leave the United States for travel to the country's region to join unspecified underground groups."
Some US lawmakers allege that it was Tamerlan's trip to Russia that ultimately led to last week's events, and questioned why the FBI did not pay more attention to the suspect after his return.
Dagestan is the most restive of the North Caucasus regions, a mountainous territory in the south with a mostly Muslim population and an ongoing Islamist, anti-Kremlin insurgency.
However the local branch of the main Caucasus Islamist group Caucasus Emirate denied on the weekend that it had any involvement in the Boston blasts, which killed three people and wounded 180.
"We are only fighting Russia, which is not only responsible for the occupation of the Caucasus, but also for monstrous crimes against Muslims," the militants said in a statement posted on the website Kavkazcenter.com