By Scott Malone

BOSTON, Jan 7 (Reuters) - Jury selection for the trial of accused Boston Marathon bomber Dzhokhar Tsarnaev will move into a more difficult phase on Wednesday after the court wraps up its initial screening of 1,200 candidates summoned to appear this week.

The field needs to be winnowed down to 12 jurors and six alternates, a challenge made all the greater by the possibility of the death penalty if Tsarnaev, 21, is convicted.

Federal law requires that all jurors chosen be willing to consider voting to put the ethnic Chechen to death, a limited population in a state that has not seen an execution in more than a half-century and where a majority of residents say they oppose the death penalty.

As he addressed each group of jurors to report to court this week, U.S. District Judge George O'Toole underlined the complexity of the task the jury will face. Tsarnaev is charged with killing three people and injuring more than 260 in the April 15, 2013 attack and with shooting dead a university police officer three days later.

"Usually, after a jury has convicted a defendant of a crime, the presiding judge decides on what the punishment should be," O'Toole told prospective jurors. "If, after trial, he is convicted of any of these crimes ... it is the responsibility of the jury, and not the judge, to decide whether Mr. Tsarnaev is to be sentenced to death."

It has been more than 30 years since Massachusetts lawmakers abolished the death penalty for crimes prosecuted in state courts. The last execution in Massachusetts was in 1947 when a pair of men convicted of murdering a former Marine died in the electric chair.

The jury hearing Tsarnaev's trial will be the third in Massachusetts to consider a death penalty case in recent decades. In 2001, a jury found a nurse guilty of killing patients but did not sentence her to die.

In 2003, a jury sentenced admitted serial killer Gary Lee Sampson to death, but that decision was overturned in 2011 after a judge found one of the jurors had lied during the screening process. Sampson is still awaiting a second trial to determine if he will face execution.

The knowledge that an error in the jury selection process can lead to a jury's decision being thrown out is powerful motivation for any judge to move carefully.

"It's a difficult enough process as it is," said David Weinstein, an attorney in private practice who in prior jobs as a state and federal prosecutor brought death-penalty cases. "The whole rest of the trial could be clean and if there was one error early on in jury selection, there's a risk it could all get turned around."

VIEWS CHANGE ON DEATH

The passions that surround the attack, which saw bombs rip through a crowd of hundreds of people at the finish line of the city's best-attended sporting event, claiming victims including an 8-year-old boy, could sway potential jurors who normally oppose the death penalty.

"There are going to be some people who say, 'Generally, I'm not in favor of the death penalty but if it's an ordinary crime, but this is no ordinary crime,'" said John Blume, director of Cornell University's Death Penalty Project.

Former Boston Mayor Thomas Menino, who throughout his two decades in office had opposed the death penalty, told reporters a week after the attack that he would reconsider for this case.

He was not alone. An April 2014 poll by the University of Massachusetts found that while 37 percent of residents supported the death penalty in general, 59 percent of respondents approved of executing Tsarnaev if he is found guilty.

As lawyers examine the juror questionnaires filled out this week, one of their first goals will be to exclude those who have an unwavering opposition to capital punishment or who have already determined that Tsarnaev should be put to death.

They will also seek to eliminate potential jurors with a close connection to the case, those who were present at the blast site or who had close friends who were injured.

After that, the court will begin next Thursday going through the time-consuming process of interviewing jurors in person, with O'Toole planning on seeing 40 per day. That process could well extend past his initial goal of starting testimony by Jan. 26, legal experts said.

"That seems unrealistically short," Blume said. "Three weeks, to me, would be maybe enough in an ordinary capital case, but I don't see this as an ordinary case." (Reporting by Scott Malone; Editing by Grant McCool)

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