The comments came as closing arguments commenced at the trial of Dzhokhar Tsarnaev, who allegedly perpetrated the bombings.
Judge George O’Toole outlined the law to the 18-person jury ahead of last-ditch arguments from the lawyers.
If found guilty, Dzhokhar Tsarnaev, who is charged with 30 counts, will face life imprisonment or execution.
His lawyers admit he carried out the attacks but say he was under the influence of his radicalized brother.
Prosecutors are expected to remind the jury of the attack’s brutality.
Judge George O’Toole explained to the jury how the 30 separate counts were related to the different elements involved – the bombings during the race, the murder of a police officer, a car jacking and a shoot-out with police.
Three people, including an 8-year-old boy, died after two pressure cooker bombs packed with nails, ball bearings and other shrapnel detonated in April 2013.
More than 260 people were injured, with many losing limbs.
Earlier in the trial, the defense made the surprise admission that Dzhokhar Tsarnaev, now 21, had participated in the attacks.
Defense lawyers argue that Tamerlan Tsarnaev, who died during a massive manhunt, had orchestrated the attacks and by doing so they hope to spare their client the death penalty.
If convicted, a second phase will determine the punishment, and the jury will have to decide whether Dzhokhar Tsarnaev will be put to death.
Boston Marathon attacks were the deadliest terror attack on US soil since 9/11.
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