Potential jurors stared intently at Boston Marathon bombing defendant Dzhokhar Tsarnaev as jury selection began under tight security Monday in what could be the nation's most closely watched terror trial since the Oklahoma City bombing two decades ago.

Tsarnaev, flanked by his attorneys, sat at a table at the front of the jury assembly room. Wearing a dark sweater and khaki pants, he looked down much of the time but occasionally glanced at the potential jurors and the judge. He also picked at his shaggy beard.

When U.S. District Judge George O'Toole Jr. introduced him to the first group of prospective jurors and asked him to stand, he acknowledged them with a slight nod.

Tsarnaev, 21, is accused of planning and carrying out the twin pressure-cooker bombings that killed three people and wounded more than 260 near the finish line of the race on April 15, 2013.

Over the next three days, a larger-than-normal pool of about 1,200 people will be summoned to federal court to be considered as potential jurors.

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The first two groups of 200 people each were given initial instructions Monday by O'Toole and then began filling out long questionnaires.

Twelve jurors and six alternates will ultimately be selected. The judge said testimony in the trial will begin on Jan. 26 and last three to four months.

O'Toole briefly outlined the 30 charges against Tsarnaev, which include using a weapon of mass destruction. He is also accused of killing an MIT police officer as he and his brother, now dead, tried to flee days after the bombings.

The jury will be asked to decide both whether Tsarnaev is guilty and, if he is convicted, what his punishment will be: life in prison or death.

Dozens of police officers were posted inside and outside the courthouse. One bombing victim, Karen Brassard, was outside the jury room, waiting to observe jury selection.

There were no Tsarnaev supporters outside the courthouse as there have been during pretrial hearings, but one man stood holding a sign calling for federal officials to be held accountable for failing to prevent the bombing.

"I'm not a supporter of his," said Kevin O'Connell, a delivery driver from Boston. "But I think the Department of Homeland Security and the FBI need to be held accountable. ... They screwed up big-time by not preventing it."

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 but was spared a jail sentence.

"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?"

The juror questionnaires that will be used to weed out people with potential conflicts. Eventually, lawyers for the government and Tsarnaev, along with the judge, will question potential jurors individually.

The questionnaire was sealed by the judge and details of what the prospective jurors were asked were not made public.

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

"I don't know whether I'll ever get any answer to that question, but I guess I want to understand what the thought process was," Abbott said. "Why he would want to do this to people ... it's really hard to understand."

The trial is perhaps the most scrutinized case of its kind since Timothy McVeigh was convicted and executed for the 1995 Oklahoma City bombing.

Tsarnaev's lawyers tried for months to get the trial moved, arguing the Boston jury pool was tainted because of the number of locals with connections to the race. They drew parallels to the McVeigh case, which was moved to Denver. But the judge refused.

Jury selection is expected to be a long process because of the need to weed out people unduly influenced by the heavy news coverage and the large number of runners, spectators and others affected by the bombings. The process also could be slowed if potential jurors express objections to the death penalty.

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. actions in Muslim countries. Tamerlan, 26, died after a firefight with police days after the bombings.

The defense is expected to argue that Dzhokhar had a difficult childhood and was heavily influenced by his elder brother, who authorities believe became radicalized in the last few years of his life.