The man, who CNN is describing only as Danny because of privacy and safety concerns, said there was a difference of personalities between the brothers.
Tamerlan Tsarnaev kept talking throughout the ordeal, barking out orders, detailing his hatred of Americans and asking Danny to remain calm.
"If you cooperate, I won't kill you," said Tamerlan Tsarnaev, according to Danny.
While the talkative Tamerlan took the lead during the carjacking, the quiet Dzhokhar responded to multiple orders, such as getting money from an ATM, he said.
Tsarni has claimed that a man known as "Misha" was responsible for radicalizing Tamerlan Tsarnaev. On Monday, a U.S. government official told CNN that FBI agents had interviewed Misha.
The man, whose real name is Mikhail Allakhverdov, denied ever encouraging a violent take on Islam and says he was not Tamerlan's teacher, according to a New York Review of Books writer who says he interviewed Misha.
Federal agents are also looking into possible links between Tamerlan Tsarnaev and a Canadian boxer-turned-jihadist killed by Russian troops in 2012, a source being briefed on the investigation said. William Plotnikov and six others died in a firefight with Russian forces in Dagestan in July 2012, while Tsarnaev was visiting the region, the source said.
Of the scores of people injured in the blasts, 12 remain in five Boston-area hospitals, though none is in critical condition, according to information the hospitals provided to CNN.
Bombing survivor Jarrod Clowery told a news conference how impressed he was by the first responders who helped him and so many others and how coordinated they were -- even better than the best drive by New England Patriots quarterback Tom Brady.
"No disrespect to Tom Brady, I love him, but he can't hold a candle to [those] people," he said.
One of the biggest questions remaining is whether federal intelligence officials could have done more to prevent the attacks.
Russian authorities twice alerted U.S. authorities that Tamerlan Tsarnaev and his mother, Zubeidat, may have become radical Islamists.
The two were entered into a terror related database in the United States.
"Based on what I can see so far, the FBI performed its duties. The Department of Homeland Security did what it was supposed to be doing," President Barack Obama said this week. "But this is hard stuff."
An independent review by the Intelligence Community inspector general is underway.