The surviving Boston bomber, Dzhokhar Tsarnaev, has told investigators he and his elder brother were angry over the US wars in Iraq and Afghanistan.
And they were largely self-radicalized by hardline Islamic views. Though speculation continues about outside help, investigators have not confirmed anything yet. US media quoted official sources saying "these are persons operating inside the United States without a nexus". The two brothers were basically "self-radicalized".
In individual only known as "Misha" is understood to have played some role in Tamerlan's radicalization, according to relatives of the brothers.
But there has been no word on it from the authorities.
Three people were killed and over 250 injured in bombings Tsarnaev and his elder brother, Tamerlan Tsarnaev, at the finish line of the Boston marathon on April 15. The elder Tsarnaev was killed in a shootout with law enforcement officials early Friday morning and Dzhorkhar was captured from a community outside Boston.
The Tsarnaev brothers' mother Zubeidat Tsarnaeva has alleged they were framed. But other relatives have spoken about Tamerlan's radicalization.
US investigators are in Dagestan, Russia, talking to the parents, who are both there but separated.