The remaining suspect in the Boston Marathon bombings has recovered enough to walk and assured his parents in a phone conversation that he and his slain brother were innocent, their mother said Thursday.

Meanwhile, the father of a Chechen immigrant killed in Florida while being interrogated by the FBI about his ties to the slain brother maintained that U.S. agents killed his son “execution-style.”

Dzhokhar Tsarnaev, 19, walked without a wheelchair to speak to his mother Zubeidat Tsarnaeva last week for the first and only phone conversation they have had since he has been in custody, she said.

In a rare glimpse at Dzhokhar Tsarnaev’s state of mind, he told her he was getting better and that he had a very good doctor, but was struggling to understand what happened, she said.

“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.

The April 15 bombings killed three people and wounded more than 260. Elder brother, 26-year-old Tamerlan Tsarnaev, was killed in the shootout with police, and Dzhokhar remains in a prison hospital after being severely wounded.

“I could just feel that he was being driven crazy by the unfairness that happened to us, that they killed our innocent Tamerlan,” their mother said, standing by the family’s insistence that their children are innocent.

Elsewhere, at a news conference in Moscow, the father of a 27-year-old mixed martial arts fighter who was killed during FBI questioning accused agents of being “bandits” and said they executed his son, Ibragim.

Abdul-Baki Todashev showed journalists 16 photographs that he said were of his son, Ibragim Todashev, in a Florida morgue. He said Ibragim had six gunshot wounds to his torso and one to the back of his head.

The FBI says Ibragim was being questioned by an FBI agent and two Massachusetts state troopers about his ties to Tamerlan Tsarnaev, as well as about a 2011 triple slaying in Massachusetts.

The officials said initially that Ibragim had lunged at the FBI agent with a knife, although two of them later said it was no longer clear what had happened.

Todashev said that his son moved to the U.S. in 2008 on a study exchange program and met Tsarnaev at a boxing gym in Boston in 2011, about a year before he moved to Orlando. He said the two were “not particularly close friends.”

Prior to last month’s bombings, Ibragim underwent an operation for a sports injury and was on crutches, his father said. He added that Ibragim had recently received a green card and was planning to return to Chechnya for the summer last Friday, two days after he was killed.

FBI agents interrogated Ibragim twice before the night he was shot, his father said. He said his son told him that he thought Tsarnaev had been set up to take the blame for the bombings.

“I’d only seen and heard things like that in the movies — they shoot somebody and then a shot in the head to make sure,” Todashev said. “These just aren’t FBI agents, they’re bandits,” he added.

The FBI wouldn’t comment on the claims made by Ibragim’s father.

The Tsarnaevs’ parents have held fast to their belief that their sons were framed. Zubeidat Tsarnaeva, dressed all in black and still visibly distressed, showed reporters several YouTube videos on an iPad she claimed cleared her sons.

She said Tamerlan told her about Ibragim, and that she and her husband had invited him to visit them in Russia, though he never came. Tamerlan later told them that he and Ibragim were unlikely to continue training together since they practiced different sports, and he appeared to have lost track of him after Ibragim moved to Florida.