Mr Tsarnaev faces a possible death sentence if convicted
Surviving Boston Marathon bombing suspect Dzhokhar Tsarnaev has recovered enough to walk, his mother says.
In an interview for the Associated Press, Zubeidat Tsarnaeva said her son told her in a phone call that he and his brother, killed in a police manhunt after the blasts, were innocent.
Mr Tsarnaev, who was shot and injured during the manhunt, is currently being held in a prison hospital.
Last month's bombings left three people dead and more than 260 others injured.
Mrs Tsarnaeva told AP that it was the first conversation she had had with her 19-year-old son since he has been held in custody.
He told her he was getting better but was struggling to comprehend what had happened.
"He didn't hold back his emotions either, as if he were screaming to the whole world: 'What is this? What's happening?'" she said.
"I could just feel that he was being driven crazy by the unfairness that happened to us, that they killed our innocent Tamerlan."
The Tsarnaev family has continued to claim the men's innocence in the bomb attacks, which targeted the finishing line of the Boston Marathon on 15 April.
The family, who are ethnic Chechen Muslims from Russia, spoke from an apartment in the Russian republic of Dagestan which reportedly belonged to 26-year-old Tamerlan, who was hit in a shoot-out with police in the aftermath of the bombings.
The suspects' father, Anzor, said they bought the apartment in anticipation of Tamerlan and his family moving to Makhachkala, the capital of Dagestan.
"All I can do is pray to God and hope that one day fairness will win out, our children will be cleared, and we will at least get Dzhokhar back, crippled, but at least alive,'' he told AP.
Meanwhile, the father of a Chechen immigrant who was shot and killed during a violent confrontation with Boston Marathon investigators has accused agents who killed his son of being "bandits".
Ibragim Todashev, 27, admitted a role in a triple murder near Boston in 2011 and implicated Tamerlan Tsarnaev in the crime.
However, no evidence has emerged to link Mr Todashev to the bombings.
On 22 May, he was shot and killed in Orlando, Florida.
Speaking at a news conference on Thursday, his father, Abdul-Baki, showed 16 photographs that he said were of his son in a Florida morgue.
He claimed his son had six gunshot wounds to his torso and one to the back of his head. However, the photos have not been authenticated.
There are conflicting reports about the events that led to Todashev's death, with law enforcement officials initially saying he was shot after attacking an FBI agent with a knife but later saying they were not clear about what happened.
Ibragim's father says his son was "100% unarmed" and has called for an investigation into his death.
"These are not FBI agents but bandits - I cannot call them anything else and they must be tried," he said.
His son met Tamerlan Tsarnaev at a boxing gym in Boston in 2011 but they were "not particularly close friends", he adds.