"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 it 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 book "The Protocols of the Elders of Zion," the classic anti-Semitic hoax, first published in Russia in 1903, that claims 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."
Associated Press writers Lara Jakes and Eileen Sullivan contributed to this report.
AP's Washington investigative team can be reached at DCinvestigations(at)ap.org