BOSTON — As some prospective jurors got their first look at Dzhokhar Tsarnaev, the young man accused of bombing the renowned Boston Marathon, it became clear that choosing a jury in the closely watched terror case would not be a quick or simple process.

U.S. District Judge George O’Toole Jr. told the first 400 people called for jury duty Monday that choosing a panel of 12 jurors and six alternates for the highly anticipated trial was expected to take at least three weeks.

He told the potential jurors not to think of the trial as “an annoying burden,” but as a needed service and an “important duty of citizenship.”

Tsarnaev is accused of planning and carrying out the twin bombings that killed three people and wounded more than 260 others near the marathon’s finish line on April 15, 2013. Prosecutors say Tsarnaev and his brother, now dead, also killed an MIT police officer several days after the bombings as they tried to flee.

Federal prosecutors and Tsarnaev’s lawyers are beginning the process of trying to find jurors who can be fair and impartial. Jurors must also be willing, if Tsarnaev is convicted, to consider imposing the death penalty in a state that abolished its own death penalty three decades ago. Tsarnaev is being tried under the federal death penalty.

Defense lawyers have asked repeatedly that the trial be moved from Boston, where the bombings had a deep emotional impact. O’Toole has refused so far.

Potential jurors Monday seemed transfixed by the sight of the shaggy-haired 21-year-old, staring at him intently when he walked in and when he was introduced by the judge.

Tsarnaev, for his part, rose to his feet and nodded, slightly and awkwardly, as he was introduced to the first group of about 200 citizens. He looked at the floor when introduced to a second group of 200.

Security was tight, with dozens of police officers stationed inside and outside the federal courthouse, along with bomb-sniffing dogs.

The first two groups of potential jurors filled out juror questionnaires, which will be used to whittle down an overall pool of about 1,200 people. Another 800 also will be asked to fill out questionnaires on Tuesday and Wednesday. Tsarnaev’s lawyers, prosecutors and the judge will begin questioning individual jurors next week.

O’Toole said testimony is expected to begin Jan. 26 and last three to four months. The trial is perhaps the most scrutinized terror case in the U.S. since Timothy McVeigh was convicted and executed for the 1995 Oklahoma City bombing.

Heather Abbott, of Newport, Rhode Island, who lost her left leg below the knee in the Boston attack, said she plans to attend some of the proceedings. She said her biggest question may be an unanswerable one: Why?

“Why he would want to do this to people … it’s really hard to understand,” Abbott said.

In Russia, the father of the Tsarnaev brothers again expressed the family’s distrust of the U.S. legal system. Recently, one of Dzhokhar Tsarnaev’s sisters pleaded guilty in Boston to misleading police during a counterfeiting investigation.

“All the information that can refute the allegations against my sons is on the Internet,” Anzor Tsarnaev said by telephone from Grozny, the capital of Chechnya. “I still have children in America and I am afraid for them. As you all know, they also caused problems for my younger daughter with fabricated allegations. Who knows what they could do with my other children?”

Prosecutors say Dzhokhar and his brother, Tamerlan Tsarnaev ”“ ethnic Chechens who had lived in the United States for about a decade ”“ carried out the bombings in retaliation for U.S. wars in Muslim countries. Tamerlan, 26, died in a gunbattle with police days after the bombings.

The defense is expected to argue that Dzhokhar had a difficult childhood and fell under the evil influence of his older brother.

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