BOSTON — People who lost loved ones or were injured in the 2013 Boston Marathon bombings are clamoring for justice as jury selection gets underway in one of the nation’s most closely watched federal death penalty cases in two decades.

Seating jurors in the case against Dzhokhar Tsarnaev was set to begin today under tight security at the federal courthouse in Boston and could take several weeks.

Jurors will be chosen from a pool of about 1,200. They will decide whether Tsarnaev planned and carried out the twin bombings that killed three people and injured more than 260 near the finish line of the race on April 15, 2013.

If they find him guilty, they also will decide whether he should be put to death. Survivors and first responders are among those expected to testify.

Heather Abbott of Newport, Rhode Island, who lost her left leg below the knee in the attacks, said her biggest question may be an unanswerable one: “Why?”