Federal prosecutors will begin their bid to convince jurors that Dylann Storm Roof, the man who gunned down nine parishioners of Charleston's historic Mother Emanuel church in hopes of starting a race war, should be sentenced to death.

Roof, 22, was convicted last month of federal hate crimes, and because prosecutors are seeking a death sentence, the same men and women who declared him guilty of 33 criminal charges must now decide his punishment. The so-called penalty phase of any trial in which a person's life is on the line is difficult to forecast, but in Roof's case, it is especially hard to predict.

Roof has asked that the lawyers who worked on his behalf during the trial's guilt phase no longer represent him, apparently because of a disagreement over whether mental health evidence should be offered. He reportedly plans to offer an opening statement, though he has said he does not plan to offer evidence or call witnesses. Today, at the request of Roof's former defence lawyer, a judge conducted a non-public hearing to assess Roof's competency, though the judge indicated before the hearing began he did not anticipate delaying the start of the penalty phase.

Over the next few days, Roof could question the witnesses called by prosecutors, including the relatives of those he killed. Or he could mostly remain silent.

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"I think people are afraid that this mentally impaired guy is going to get up and turn it into a circus," said Christopher Adams, a Charleston defence lawyer who is not involved in the case. "But what I suspect will happen is he just won't do anything - that he'll watch the witnesses, that he won't put up any meaningful evidence on his own."

Jurors have only two options: sentence Roof to life in prison without the possibility of release or consign him to die. Prosecutors will set out to convince them that the case presents the factors necessary to warrant the most severe penalty in the American justice system. The Justice Department is seeking a death sentence for more than half of the 33 counts Roof was convicted of last month.

Federal death sentences are a rarity. There are 59 federal inmates facing such sentences, according to the Bureau of Prisons. The most recent addition was Dzhokhar Tsarnaev, sentenced to death in 2015 for his role in the Boston Marathon bombings. Since the federal death penalty statute was reinstated in 1988 and expanded in 1994, the Government has taken a little more than 200 such cases to trial, according to the Federal Death Penalty Resource Counsel Project. Juries deciding sentences handed down punishments of life in prison about twice as often as the death penalty.

The Justice Department has executed only three inmates in the modern death penalty era, and the last such execution was in 2003.

When they revealed to the court in May that they would seek the death penalty - a decision that was opposed even by then-US Attorney Bill Nettles and Vanita Gupta, the Justice Department's assistant attorney general for the civil rights division - prosecutors highlighted Roof's substantial planning and premeditation of the crime, his racial motivation, his lack of remorse and the vulnerability of his victims. The federal death penalty statute states that the punishment is more warranted when a victim is "particularly vulnerable" because of being notably young or old; three of the Mother Emanuel victims were between the ages of 70 and 87.

Jurors already have seen haunting evidence about the nature of the crime and the impact it had on the families' of those whose loved ones were killed. In their opening statement, prosecutors flashed pictures of each victim and outlined brief biographies of the lives that were taken.

Jurors would later see a video of Roof confessing to the crime - "Well, I killed them, I guess," he said - and offering a racist rationale.

"I had to do it because somebody had to do something," Roof said. "Black people are killing white people every day on the street, and they are raping white women. What I did is so minuscule to what they're doing to white people every day all the time."

For his part, Roof could present what is known as mitigating evidence, or reasons that jurors would not want to see him executed. That might have included testimony about his mental health evidence, though Roof told a court last month he would call no experts and offer no evidence on that subject. Adams said Roof could also tell jurors about his life and background, and how miserable his existence might be in a federal prison, in hopes that it might convince the group to spare his life.

But Adams said Roof would face a tall order.

"This work is really hard. It's very technical, and it involves a lot of emotion and a lot of technical skill," Adams said. "For a guy who's a high school dropout, it's almost impossible. And I think, for any defendant, no matter how smart, it would be impossible to hit the right emotional tones for the jurors."

Roof had offered to plead guilty in exchange for a life sentence - an indication that he does not likely want to die, as some facing the prospect of life behind bars do.

If jurors sentence him to life in prison, he faces the possibility of another death sentence at his state trial, expected to begin later this year. But South Carolina has not executed a death row inmate since 2011. Like many other states, it has no lethal injection drugs, and it does not appear likely it will obtain any in the immediate future.