By Elizabeth Barber

BOSTON (Reuters) - The first full day of jury deliberations in the Boston Marathon bombing trial ended on Thursday without a decision on whether Dzhokhar Tsarnaev would be sentenced to death or to life in prison for the deadly 2013 attack.

The same 12 jurors who found the 21-year-old guilty in April of killing three people and wounding 264 at the race's crowded finish line, have been instructed to weigh a list of aggravating and mitigating factors reprising the trial's key themes.

In the prosecution's view, Tsarnaev is an unrepentant mass killer. The defense has painted him as a hapless college kid who was influenced by his 26-year-old brother, Tamerlan.

Prosecutors argued that Tsarnaev is a jihadist who chose the world-renowned race as the best place to kill and maim as many people as possible, including children, for whom the day is a school holiday.

They said the defendant has shown no remorse for crimes he justified as vengeance for U.S. military campaigns in Muslim-dominated countries.

Tsarnaev was convicted of murdering 8-year-old Martin Richard, 23-year-old Chinese graduate student Lingzi Lu, 29-year-old restaurant manager Krystle Campbell and Massachusetts Institute of Technology policeman Sean Collier.

'PERSONAL INTENT'

After a few hours of deliberations, the jury asked how to apply the legal concept of "aiding and abetting" while weighing Tsarnaev's sentence.

That is a key issue in the trial, since Dzhokhar Tsarnaev has been found guilty both of directly killing people by placing the bomb that killed Richard and Lu, as well as of causing Campbell's death with a bomb that Tamerlan had planted.

U.S. District Judge George O'Toole told the jurors to consider whether Dzhokhar intended to kill people.

He instructed them that their decision "must be based on Mr. Tsarnaev's personal actions and intent, and not on the actions and intent of anyone else."

Defense lawyers contend that Tsarnaev was a bright, gentle kid who ultimately could not overcome his circumstances: a mentally ill father and inattentive mother whose general neglect became acute when they left him in the United States to return to Russia in 2012, and a radical older brother, who, they contend, was the architect of the bombings.

The jurors have been asked to return a sentence for each of the 17 death penalty-eligible crimes. A death sentence for any offense would supersede all life sentences. All 12 jurors must reach unanimous agreement to sentence Tsarnaev to death.

(Editing by Scott Malone, Bernadette Baum and Jonathan Oatis)