Under the tutelage of a friend known to the Tsarnaev family only as Misha, Tamerlan gave up boxing and stopped studying music, his family said. He began opposing the wars in Afghanistan and Iraq. He turned to websites and literature claiming that the CIA was behind the terrorist attacks of Sept. 11, 2001, and Jews controlled the world.
"Somehow, he just took his brain," said Tamerlan's uncle, Ruslan Tsarni, who recalled conversations with Tamerlan's worried father about Misha's influence. Efforts over several days by The Associated Press to identify and interview Misha have been unsuccessful.
Tamerlan's relationship with Misha could be a clue in understanding the motives behind his religious transformation and, ultimately, the attack itself. Two U.S. officials say he had no tie to terrorist groups.
Throughout his religious makeover, Tamerlan maintained a strong influence over his siblings, including Dzhokhar, who investigators say carried out the deadly attack by his older brother's side, killing three and injuring 264 people.
"They all loved Tamerlan. He was the eldest one and he, in many ways, was the role model for his sisters and his brother," said Elmirza Khozhugov, 26, the ex-husband of Tamerlan's sister, Ailina. "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.
"Even my ex-wife loved him so much and respected him so much,"said Khozhugov. "I'd have arguments with her and if Tamerlan took my side, she would agree: 'ok, if Tamerlan said it.'"
Khozhugov said he was close to Tamaran when he was married and they kept in touch for a while but drifted apart in the past two years or so. He spoke to the AP from his home in Almaty, Kazakhstan. A family member in the United States provided the contact information.
Tamerlan Tsarnaev was killed in a police shootout Friday. Dzhokhar Tsarnaev was charged Monday with using a weapon of mass destruction to kill, and he could face the death penalty if convicted.
"Of course I was shocked and surprised that he was Suspect No. 1," Khozhugov said, recalling the days after the bombing when the FBI identified Tamerlan as the primary suspect. "But after a few hours of thinking about it, I thought it could be possible that he did it."
Based on preliminary written interviews with Dzhokar in his hospital bed, U.S. officials believe the brothers were motivated by their religious views. It has not been clear, however, what those views were.
As authorities try to piece together that information, they are touching on a question asked after so many terrorist plots: What turns someone into a terrorist?
The brothers emigrated in 2002 or 2003 from Dagestan, a Russian republic that has become an epicenter of the Islamic insurgency that spilled over from the region of Chechnya.
They were raised in a home that followed Sunni Islam, the religion's largest sect. They were not regulars at the mosque and rarely discussed religion, Khozhugov said.
Then, in 2008 or 2009, Tamerlan met Misha, a slightly older, heavyset bald man with a long reddish beard. Khozhugov didn't know where they'd met but believed they attended a Boston-area mosque together. Misha was an Armenian native and a convert to Islam and quickly began influencing his new friend, family members said.
Once, Khozhugov said, Misha came to the family home outside Boston and sat in the kitchen, chatting with Tamerlan for hours.
"Misha was telling him what is Islam, what is good in Islam, what is bad in Islam," said Khozhugov, who said he was present for the conversation. "This is the best religion and that's it. Mohammed said this and Mohammed said that."
The conversation continued until Tamerlan's father, Anzor, came home from work.
"It was late, like midnight," Khozhugov said. "His father comes in and says, 'Why is Misha here so late and still in our house?" he asked politely. Tamerlan was so much into the conversation he didn't listen."
Khozhugov said Tamerlan's mother, Zubeidat, told him not to worry.
"'Don't interrupt them,'" Khozhugov recalled the mother saying. "They're talking about religion and good things. Misha is teaching him to be good and nice."
As time went on, Tamerlan and his father argued about the young man's new beliefs.
"When Misha would start talking, Tamerlan would stop talking and listen. It upset his father because Tamerlan wouldn't listen to him as much," Khozhugov said. "He would listen to this guy from the mosque who was preaching to him."
Anzor became so concerned that he called his brother, worried about Misha's effects.
"I heard about nobody else but this convert," Tsarni said. "The seed for changing his views was planted right there in Cambridge."
It was not immediately clear whether the FBI has spoken to Misha or was attempting to.
Tsarnaev became an ardent reader of jihadist websites and extremist propaganda, two U.S. officials said. He read Inspire magazine, an English-language online publication produced by al-Qaida's Yemen affiliate.
Tamerlan loved music and, a few years ago, he sent Khozhugov a song he'd composed in English and Russian. He said he was about to start music school.
Six weeks later, the two men spoke on the phone. Khozhugov asked how school was going.
"I quit," Tamerlan said.
"Why did you quit?" Khozhugov asked. "You just started."
"Music is not really supported in Islam," he replied.
"Who told you that?"
"Misha said it's not really good to create music. It's not really good to listen to music," Tamerlan said, according to Khozhugov.
Tamerlan took an interest in Infowars, a conspiracy theory website. Khozhugov said Tamerlan was interested in finding a copy of "The Protocols of the Learned Elders of Zion," a piece of anti-Semitic literature claiming a Jewish plot to take over the world.
"He never said he hated America or he hated the Jews," Khozhugov said. "But he was fairly aggressive toward the policies of the U.S. toward countries with Muslim populations. He disliked the wars."
One of the brothers' neighbors, Albrecht Ammon, recently recalled an encounter in which Tamerlan argued about U.S. foreign policy, the wars in Afghanistan and Iraq, and religion.
Ammon said Tamerlan described the Bible as a "cheap copy" of the Quran, used to justify wars with other countries.
"He had nothing against the American people," Ammon said. "He had something against the American government."
Khozhugov said Tamerlan did not know much about Islam beyond what he found online or what he heard from Misha.
"Misha was important," he said. "Tamerlan was searching for something. He was searching for something out there."