Family members reached in the U.S. and abroad by The Associated Press said Tamerlan was steered toward a strict strain of Islam under the influence of a Muslim convert known to the Tsarnaev family only as Misha.
After befriending Misha, Tamerlan gave up boxing, stopped studying music and began opposing the wars in Afghanistan and Iraq, according to family members, who said he turned to websites and literature claiming that the CIA was behind 9/11.
"Somehow, he just took his brain," said Tamerlan's uncle, Ruslan Tsarni of Montgomery Village, Md., who recalled conversations with Tamerlan's worried father about Misha's influence.
"You could always hear his younger brother and sisters say, 'Tamerlan said this,' and 'Tamerlan said that.' Dzhokhar loved him. He would do whatever Tamerlan would say," recalled Elmirza Khozhugov, the ex-husband of Tamerlan's sister. He spoke by telephone from his home in Almaty, Kazakhstan.
The brothers, who came to the U.S. from Russia a decade ago, were raised in a home that followed Sunni Islam, the religion's largest sect, but were not regulars at the mosque and rarely discussed religion, Khozhugov said.
Then, in 2008 or 2009, Tamerlan met Misha, a heavyset bald man with a reddish beard. Khozhugov didn't know where they met but believed they attended a Boston-area mosque together.
Hoping to learn more about the Tsarnaev brothers' motives, U.S. investigators traveled to southern Russia on Tuesday to speak to their parents, a U.S. Embassy official said.
A lawyer for the family, Zaurbek Sadakhanov, said the parents had just seen pictures of the mutilated body of their elder son and were not up to speaking with anyone.
In Massachusetts, the state House turned aside a bid by several lawmakers to reinstate the death penalty in certain cases, including the murder of police officers. In a 119-38 vote, the House sent the proposal to a study committee rather than advance it to an up-or-down vote.
In another development, April Walton, the manager of Phantom Fireworks of Seabrook, N.H., said Tamerlan Tsarnaev bought 48 mortar shells at the store in February.
Company Vice President William Weimer said FBI agents visited the store on Friday, interviewed staff and checked its computers. He said the amount of gunpowder that could be extracted from the fireworks would not have been enough for the Boston bombs.
Associated Press writers Bridget Murphy and Bob Salsberg in Boston, Lynn Berry in Moscow, and Adam Goldman, Eric Tucker, Matt Apuzzo, Kimberly Dozier and Eileen Sullivan in Washington contributed to this report.