Convicted Boston Marathon bomber Dzhokhar Tsarnaev must face death for the attack that killed three people and wounded 254, prosecutors will argue when a court this week begins weighing up his fate.

The sentencing phase of Tsarnaev's high-profile trial begins Tuesday in the city, just days after the parents of a young boy killed in the dual blasts on April 15, 2013 asked for the death penalty to be dropped.

Tsarnaev faces either life in prison or the death penalty after a jury unanimously convicted him this month of carrying out the worst attack in the United States since the 9/11 hijackings.

During sentencing, which is expected to last three or four weeks, prosecutors will try to convince the 12 jurors there are enough aggravating factors -- including premeditation, the number of victims and a lack of remorse -- to warrant capital punishment.

Defense attorneys for Tsarnaev, a Muslim immigrant of Chechen descent who became a US citizen in 2012, will argue that he was merely a confused teenager who was under the influence of his elder, radicalized brother, Tamerlan Tsarnaev.

Tamerlan, 26, was shot dead by police while on the run.

As was the case during the first part of the trial, each side will call witnesses.

"I think we'll hear a lot more from the defense about who the defendant is, his young age, what is life has been like, what his relationship with his brother was," said University of New Hampshire professor Albert Scherr, an expert on the death penalty.

- Emotional letter -

On Friday, the parents of the youngest victim, eight-year-old Martin Richard, published an open letter stating their opposition to the death penalty for Tsarnaev, who was 19 when he and his brother carried out the attack.

"We know that the government has its reasons for seeking the death penalty, but the continued pursuit of that punishment could bring years of appeals and prolong reliving the most painful day of our lives," parents Bill and Denise Richard wrote in an emotional letter published in The Boston Globe.

They referred to the misery the attack caused their family, including badly injuring their daughter, whose leg was amputated after the twin bombings.

Instead of the death penalty, the Richards want Tsarnaev sentenced to life in prison with no chance of parole.

Federal prosecutor Carmen Ortiz said she has frequently spoken to victims' families, including the Richards.

"I care deeply about their views and the views of the other victims and survivors," she said.

"As the case moves forward we will continue to do all we can to protect and vindicate those injured and those who have passed away."

Though the death penalty has been abolished in Massachusetts since 1984, prosecutors can still request the sentence because it is a federal case.

Massachusetts has not executed anyone since 1947 and Catholic bishops in the state have reiterated their opposition to the death penalty, including in this case.

But the 12 jurors in Boston were selected in part based on their openness to impose the death penalty.

"That is really a hard decision (for the jury)," Scherr said.

On the one hand, they know what a heinous crime this was, he said, but they also will be shown that Tsarnaev was "a 19-year-old kid whose decision-making brain is not fully developed."

"I don't envy them," he added.
Source: AFP