The jury was given the case Wednesday afternoon following attorneys' closing arguments. The prosecution told jurors Tsarnaev was a remorseless terrorist worthy of the death penalty, while the defense said he was repentant and deserved to be spared.
"The defendant knew what kind of hell was going to be unleashed," prosecutor Steve Mellin said.
Defense attorney Judy Clarke countered: "We're asking you to choose life. Yes, even for the Boston Marathon bomber."
U.S. District Court Judge George O'Toole sent the jury out shortly after 4 p.m. Wednesday. "The importance of your deliberations is obvious," he said. About an hour later, jurors were sent home for the night.
"You have a long way to go," the judge told them before reminding them to avoid any outside information or discussion about the case overnight.
Tsarnaev, who is 21, was convicted in April of 30 counts; 17 of them carry a possible death sentence. He was found guilty of conspiring and detonating weapons of mass destruction at a public event as an act of terrorism resulting in death.
Jurors will complete a 24-page questionnaire that asks them to weigh "aggravating" factors presented by the prosecution and "mitigating" factors advanced by the defense before making a final decision on life or death for each of the 17 counts.
O'Toole earlier reminded jurors they have to be unanimous to return a death sentence and that no juror is under any obligation to choose death.
Hoping to spare Tsarnaev's life, defense attorney Clarke told jurors in her closing argument that her client has shown repentance and is "not the same angry young man the prosecution has described to you."
A life sentence ensures that Tsarnaev will be "locked away in a bleak environment under bleak conditions
," Clarke said.
"He'll have no glory or stature that martyrdom will bring. His name will fade from the headlines. It will fade from the news altogether and those who so desperately no longer want to be reminded of him."
Clarke made her plea shortly after the prosecution told jurors Tsarnaev was a terrorist whose actions "have earned him a sentence of death."
Prosecution: 'What terrorism looks like'
In his closing argument, prosecutor Mellin showed gruesome images to remind jurors of the carnage unleashed on April 15, 2013
, stating: "This is what terrorism looks like."
Mellin then named the three people killed in the bombings carried out by Tsarnaev and his older brother, Tamerlan Tsarnaev, who was killed in a shootout with police.
, 8: "Bleeding in agony, while his mother bends over, injured in one eye, begging him to live."
, 23: "Screaming in pain while she dies in that street."
, 29: "Burned all over her body, filled with shrapnel, with smoke coming out of her mouth."
He implored jurors to remember the others in the crowd or running that day who stared "in shock at their mangled and ruined limbs."
"It's hard to think of a better place to murder people than the Boston Marathon if you want to make a statement [that] you think Americans are in need of punishment," Mellin said.
The prosecutor told jurors not to forget about the fourth person killed by the Tsarnaev brothers. Sean Collier
, a 27-year-old MIT security officer, was shot at point-blank range days after the bombing.
"Killing a police officer makes all of us more vulnerable."
He especially cited the vulnerability of Martin Richard, who weighed less than 70 pounds. "Can there be anybody more vulnerable than a little boy next to a weapon of mass destruction?" Mellin asked.
"The defendant placed that bomb on the ground, so the smaller the victims were the more exposed they were to that shrapnel."
The entire Richard family was standing there, with Martin's mother, Denise Richard
, suffering grave wounds and his younger sister, Jane, having one of her legs blown off.
"This defendant blinded their mother, maimed their 6-year-old daughter and blew apart their 8-year-old son in front of the father."
Mellin beat back at the defense portrayal of Tsarnaev as a good-natured kid who fell prey to his older brother and followed his orders.
"All murderers start out as cute children, but sometimes the cute children grow up to be bad people," Mellin said. "Tamerlan Tsarnaev was not his brother's master. They were partners in crime and brothers in arms."
He concluded his argument by saying "a death sentence is not giving him what he wants. It's giving him what he deserves."
"After all of the damage and carnage and fear he has caused, the right decision is clear. The only sentence that will do justice in this case is the sentence of death."
Defense: Blame the brother
In her closing argument, Clarke blamed the older brother, Tamerlan
, for the radicalization of a once promising, hard-working kid.
"The story of Jahar cannot be told without knowing the story of Tamerlan," Clarke said. "How does this good kid, this youngster, this young man who was described as gentle by friends and family and teachers do this? How does this happen?
"If you expect me to have an answer, a simple clean answer as to how this could happen, I don't have it," the defense attorney said. "I can tell you this. Jahar Tsarnaev is not the worst of the worst. And that is what the death penalty is reserved for -- the worst of the worst.
"Is his a life worth saving? Is there hope for him? Is there a chance for redemption?"
She reminded jurors of the testimony of Sister Helen Prejean, the Roman Catholic nun famous for counseling the condemned on death row. Prejean said she believed Tsarnaev was "genuinely sorry" for the pain and suffering he inflicted on his victims.
"That sounds like growth," Clarke told jurors Wednesday. "What unrepentant, unchanged, untouched young jihadi is going to meet with a Catholic nun. What unrepentant, hate-filled jihadi would even try to get her?"
Clarke is one of the nation's foremost experts on keeping clients off death row.
She has successfully fought for the lives of Ted Kaczynski, known as the Unabomber, and Jared Loughner
, the gunman who killed a judge and wounded former U.S. Rep. Gabrielle Giffords
In the government's rebuttal, prosecutor Bill Weinreb repeatedly called Tsarnaev a terrorist and again sought to undermine the defense's portrayal of him acting at his brother's request. "Where is the evidence that he was under his brother's spell?
"The defendant deserves the death penalty -- not because he's inhuman but because he's inhumane."
He ended with: "If you sentence him to life in prison, you will be giving him the minimum punishment under law. Ask yourselves: Do these crimes deserve the minimum punishment?"