(AP) -- Some of the prospective jurors who could decide Dzhokhar Tsarnaev's fate got their first look Monday at the young man accused of bombing the Boston Marathon, and they seemed transfixed by the sight of the shaggy-haired 21-year-old.

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.

So began what could be weeks of jury selection in the nation's most closely watched terror trial since the Oklahoma City bombing two decades ago.

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

The potential jurors seemed riveted by Tsarnaev and by U.S. District Judge George O'Toole Jr.'s explanation of the gravity of what they will be asked to do if they are picked: They must decide not only whether the former college student is guilty or innocent, but also what his punishment will be if he is convicted - life in prison or execution.

The judge 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 pressure-cooker bombings that killed three people and wounded more than 260 near the finish line of the race on April 15, 2013.

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, made their getaway.

Tsarnaev, flanked by his attorneys, sat at a table at the front of the room. Wearing a dark sweater and khaki pants, he looked down much of the time and picked at his beard.

He nodded to the first group of potential jurors in the morning. When he stood for a second group of 200 in the afternoon, he looked down at the floor.

Over three days, a pool of about 1,200 prospective jurors will be summoned to court. 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.

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?

"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 unusually large pool was seen as necessary because of the need to weed out those unduly influenced by heavy news coverage of the tragedy, along with the many runners, spectators and others affected by the bombings. Also, those who are unalterably opposed to the death penalty will not be allowed on the jury.

Tsarnaev's lawyers tried and failed to get the trial moved out of Boston.

The first two groups of 200 people each were given instructions by the judge and then began filling out long questionnaires. The questionnaire was sealed by the judge. Beginning next week, lawyers for the two sides will question potential jurors individually.

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.

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.

There were no Tsarnaev supporters outside the courthouse Monday, but one man stood holding a sign calling for federal officials to be held accountable for the bombing.

"They screwed up big-time by not preventing it," said Kevin O'Connell, a delivery driver from Boston.