The testimony from Sister Helen Prejean, whose autobiography "Dead Man Walking" made her an internationally acclaimed anti-death-penalty activist, was the last defense witness to take the stand in the two-month trial. Dressed in a gray suit and white blouse, Prejean said she and Tsarnaev secretly bonded over the last two months during five jailhouse meetings.
"After we had established trust with each other, he said emphatically that no one deserves to suffer like they did," she testified. She was not pressed about whether those were Tsarnaev's exact words, but she recounted them twice using similar phrasing.
She said Tsarnaev was "absolutely sincere. His response was so spontaneous. It was not like he was hedging and I had every reason to believe it was sincere.
"His face registered it, and he kind of lowered his eyes, and it was his voice. It had pain in it when he said what he did, about how nobody deserves that. I think he was taking it in and he was genuinely sorry for what he did."
After she stepped down, prosecutors presented two final rebuttal witnesses – an FBI agent and a prison warden -- and closed their case too.
Judge George A. O'Toole Jr. told the jury of seven women and five men to return to his courtroom on Wednesday for closing statements and jury instructions in the case. He said the panel will begin deliberating Wednesday afternoon on whether he should be executed or spend the rest of his life in prison with no parole.
The government wants Tsarnaev to become the first terrorist to be executed in the U.S. in the post-9/11 era. The April 2013 bombings killed three and injured more than 260 others. The defense is hoping to convince the jury to deliver the only other alternate sentence – life in prison with no parole.
Boston is a largely Catholic community, where opposition to the death penalty runs high. There is no state death penalty statute. A single hold-out among the 12 jurors would mean an automatic life sentence.
A Louisiana nun since 1957, Prejean, a member of the Sisters of St. Joseph, described her prison ministry focusing on death row prisoners as well as victims and survivors. A cross draped around her neck, she spoke about how defense lawyers in early March asked her to meet Tsarnaev.
She said their jailhouse meetings began with discussions of Catholicism and Islam, the Bible and the Koran. She said she read and studied the Koran to prepare for their sessions, "to try to understand what we have in common, what the deep spiritual core is of both."
"I'm not sure he ever met a nun before," she said. "But he was very open and receptive. He was pleasant. It was good. I walked into the room and I looked at his face and I thought, 'My, he's so young.' It was easy to start a rapport."
Under cross examination by the government, Prejean acknowledged she has been active in raising money to fight the death penalty.
But she testified that she had taken no money from Tsarnaev's legal team to boost her campaign. "Not a dime," she said.
"Would you tell this jury he was sincerely remorseful if you did not truly believe it?" asked defense lawyer Miriam Conrad.
"No, I would not," she said.
After Prejean's testimony, the government presented a short rebuttal with two additional witnesses.
Both sides then rested their case.
Closing statements and jury instructions are expected Wednesday, with deliberations beginning that afternoon.