BOSTON — Potential jurors stared intently at Boston Marathon bombing suspect Dzhokhar Tsarnaev as jury selection in his federal death penalty trial began Monday under tight security.

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

When Judge George O’Toole Jr. introduced him and asked him to stand, he acknowledged the group with a slight nod.

Over the next three days, about 1,200 people will be called to federal court to be considered as potential jurors. The first 200 were given initial instructions Monday by O’Toole. Twelve jurors and six alternates are to be selected.

The judge said the trial will begin on Jan. 26 and will 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 tried to flee several days after the bombings.

The judge also explained that the trial is unlike most other federal trials. In this case, the jury will be asked to decide both whether Tsarnaev is guilty and what his punishment will be if he is convicted: life in prison or death.

The courthouse was under tight security Monday, with dozens of police officers inside and outside the building. 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.

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

Survivors and first responders are among those expected to testify.

The judge acknowledged that serving as a juror can be “at the very least, inconvenient,” but he said jurors will not automatically be excused if they have a hardship such as a demanding work schedule or if they have read extensively about the case.

The prospective jurors group began filling out lengthy 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, then choose the jury.