Boston (CNN) Boston Marathon bomber Dzhokhar Tsarnaev ended his long silence on Wednesday, apologizing for the pain and suffering he caused his victims before a judge formally imposed his death sentence.

"If there is any lingering doubt, let there be no more. I did it, along with my brother," Tsarnaev said, referring to the bombings carried out by him and older brother Tamerlan. "I ask Allah to have mercy on me, my brother and my family."

Tsarnaev, 21, bowed his head and clasped his hands in front as he stood at the defense table. Speaking in a low, slightly accented voice, he expressed remorse but never turned to face his victims.

He said he had come to know their names, faces and ages during his trial, but he did not address any of them by name. Two dozen victims had given powerful victim impact statements earlier in the day.

"Now, I am sorry for the lives that I've taken, for the suffering that I've caused you, for the damage that I've done. Irreparable damage," Tsarnaev said.

"Allah said in the Quran that no soul is burdened with more than it can bear, and you told us just how unbearable it was, how horrendous it was, this thing I put you through," he said. "I also wish that far more people had a chance to get up there (and speak), but I took them from you."

Addressing the survivors who packed the courtroom, Tsarnaev said he prayed for Allah "to bestow his mercy upon the deceased, those affected in the bombing and their families. I pray for your relief, for your healing, for your well-being, for your strength."

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Judge George O'Toole told Tsarnaev he had embraced a cruel God, heeded the jihadist "siren song" and engaged in "monstrous self-deception" to carry out the bombings. The judge quoted works by Shakespeare and Verdi as he formally imposed the death sentence -- a decision already made by a federal jury

"Whenever your name is mentioned, what will be remembered is the evil you have done," O'Toole said. "No one will remember that your teachers were fond of you. No one will mention that your friends found you funny and fun to be with. No one will say you were a talented athlete or that you displayed compassion in being a Best Buddy or that you showed more respect to your women friends than your male peers did.

"What will be remembered is that you murdered and maimed innocent people and that you did it willfully and intentionally. You did it on purpose."

O'Toole recalled Verdi's opera "Otello" and the evil character Iago, who tries to justify his malice by saying he believes in a cruel God.

"Surely someone who believes that God smiles on and rewards the deliberate killing and maiming of innocents believes in a cruel God," the judge said. "That is not, it cannot be, the God of Islam. Anyone who has been led to believe otherwise has been maliciously and willfully deceived."

After the sentencing, U.S. Attorney Carmen Ortiz said she was more struck by what Tsanaev didn't say, particularly his failure to denounce terrorism and Islamic extremism.

Survivor Lynn Julian told reporters outside court that Tsarnaev's "Oscar-worthy" speech lacked sincerity.

"I regret ever wanting to hear him speak," she said.

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But another survivor, Henry Borgard, 23, said he accepted the apology. He added that when he locked eyes briefly with Tsarnaev in court, he saw a boy.

"I do know that I believe in second chances," Borgard said. "The man, the boy who planted that bomb that blew up in front of me is younger than I am."

Tsarnaev is the first person to be handed a death sentence in a federal terrorism case since the September 11, 2001 , attacks. He and older brother Tamerlan, who died while fleeing police, set off two bombs at the finish line of the Boston Marathon on April 15, 2013.

Judge to #Tsarnaev: You redefined what it is to be 'innocent person.'It was monstrous self-deception. You had to forget your own humanity. — Deborah Feyerick (@DebFeyerickCNN) June 24, 2015

Two women and an 8-year-old boy were killed and more than 260 other people were injured. The blasts left 17 people -- all active, outdoorsy people -- amputees. A fourth person, an MIT police officer, was killed during the hunt for the Tsarnaevs.

It was the day of final reckoning for Tsarnaev , one also set aside for emotional impact statements from victims. With the sentence a foregone conclusion, the only lingering question had been whether Tsarnaev would break his long silence.

For most of the hearing, the families of the victims and the survivors did all the talking, leaving the 11 jurors and three alternates who attended in tears.

Many calls for peace and reconciliation at #Tsarnaev sentencing hearing — Ann O'Neill (@AnnoCNN) June 24, 2015

"I know life is hard, but the choices that you made were despicable," said Patricia Campbell, the mother of victim Krystle Campbell. She stood with her husband, William, and her son and brother as she spoke directly to Tsarnaev in court.

"You will never know why she is so desperately missed by those of us who loved her," Karen McWatters, a friend of Campbell's, told Tsarnaev, who was facing in the direction of the speakers but not directly looking at them.

Tsarnaev instead often looked down, as he did during most of his long trial.

Rebekah Gregory, who lost a leg, said she was not giving a victim impact statement.

"To do that," she said, "I would have to be someone's victim. I'm definitely not yours, or your brother's."

Defiantly speaking to #Tsarnaev, amputee Rebecca Gregory on victim impact: "I am not someone's victim, not yours, not your brother's." — Deborah Feyerick (@DebFeyerickCNN) June 24, 2015

Tsarnaev's actions served only to bring people together, she said. Soon, people will forget him as well as his brother.

"It's so funny to me that you smirk and flip off the camera because that is what I feel we do to you every day we continue to succeed, fake limbs or not," she said, referring to the infamous image of the defendant raising his middle finger to a surveillance video in his cell

"We are 'Boston Strong,' " she told Tsarnaev, who did not look at her. "We are America strong."

Survivor Jennifer Kauffman said her life was forever altered but told Tsarnaev, "I forgive you and your brother."

"My hope is someday soon you will be brave enough to take complete responsibility for your actions," she said.

Heather Abbott, a dancer who lost her left leg below the knee, wondered if Tsarnaev considered the stories of agonizing pain and suffering "success stories."

"I would like Dzhokhar Tsarnaev to know that he did not break me," she said.

Jennifer Rogers, sister of MIT officer Sean Collier, who was shot to death by the fleeing brothers, said her life is forever changed.

"There is an emptiness that I cannot manage to fill," she said. "When I'm angry I am furious, when I'm sad it is debilitating."

Moving on without her brother means starting over, she added.

"I will toast whiskey in his honor and I will cry with grown men," she said. With her father standing beside her, she said she has accepted that her family will never again be happy or whole.

#Tsarnaev Victim lost friends: "Perception is I look fine, I should be fine. On outside I look normal but on inside I'm still dealing..." — Deborah Feyerick (@DebFeyerickCNN) June 24, 2015

Bill Richard, father of 8-year-old bomb victim Martin Richard, said Tsarnaev could have changed his mind that April morning and "walked away with a minimal sense of humanity." But he didn't.

"He chose to do nothing to prevent all of this from happening," he said. "He chose hate. He chose destruction. He chose death. This is all on him," he said. "We choose love. We choose kindness. We choose peace."

Bill Richard: "He chose destruction. He chose death. This is all on him. We choose love. We choose kindness. We choose peace." #Tsarnaev — Ann O'Neill (@AnnoCNN) June 24, 2015

The Richard family had urged prosecutors to drop the death penalty as a punishment option because of the anguish it will likely cause for them to go through the lengthy appeals process. Bill Richard said the family would have preferred that Tsarnaev spend the rest of his life contemplating what he has done. That time now may be shortened by the death sentence.

"Until the day he has come to recognize what he has done, there can be no reconciliation." he said. "On the day he meets his maker, may he understand what he has done, and may justice and peace be found.

Another survivor, Jeanne-Marie Parker, said: "My only hope is that you own this grief for the rest of your natural life."

Amputee: #Tsarnaev standing behind kids "couldn't possibly have had a soul to do such a thing." Blaming brother "What a cowardly defense." — Deborah Feyerick (@DebFeyerickCNN) June 24, 2015

Marathon runner Meaghan Zipin said: "I'm the one who is alive, the defendant is already dead."

Prosecutors say the Tsarnaev brothers set off their homemade bombs -- containing fireworks, BBs, nails and metal shards packed inside pressure cookers -- to become martyrs to the cause of jihad. They also sought to punish Americans for the deaths of Muslims overseas.

Until Wednesday, Tsarnaev's only public words of explanation existed in a rambling "manifesto" scrawled with a pencil on the sides of a boat while he hid for 18 hours before surrendering to police. The message was punctuated by bullet holes and streaked with blood.

At the defense table during the trial, Tsarnaev fiddled with his bushy beard and tangle of curls, betraying no emotion as survivors and families of the dead told their heartrending stories. His face was blank as horrific images of the devastation he caused filled the screens inside the courtroom.

The only flicker of emotion came when he appeared to wipe a tear from his eye while an elderly aunt, brought from Russia by the defense, dissolved into tears and gasping sobs on the witness stand.

Sister Helen Prejean , a Catholic nun who has earned renown for her work with death-row inmates, testified for the defense that Tsarnaev seemed "genuinely sorry." She said he told her during one of several visits that the bombing victims didn't deserve to suffer.

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"He said it emphatically," she told jurors, quoting Tsarnaev as saying, "Nobody deserves to suffer like they did."

Defense attorney Judy Clarke chose her words carefully when she spoke during the trial about whether Tsarnaev felt remorse. She stopped short of telling jurors in her closing argument that he was sorry. All she'd say was he'd shown signs of "maturity" and was on the road to someday being remorseful.

In the end, only two of the 12 jurors found any glimmer of remorse.

Tsarnaev was convicted of 30 counts , and the jury determined that six specific crimes merited the death penalty. They involved the deaths of Lingzi Lu, a 23-year-old graduate student from China, and Martin Richard, the 8-year-old boy. The two were killed by the bomb Tsarnaev set off in front of the Forum, a restaurant near the marathon finish line on Boylston Street. It literally tore them apart.

The jury did not find that he should be punished by the death penalty for the death of Campbell, who was killed by the bomb set off by his brother near Marathon Sports. And they did not condemn him to die for the shooting of Collier, the MIT police officer, a few days later. Those crimes will be punished by the only other alternative: life in prison without parole.

The U.S. Marshal's Service will maintain custody of Tsarnaev until his appeals are exhausted, the judge ordered Wednesday.

Tsarnaev has been held under the tight restrictions usually reserved for terrorists; they include solitary confinement and no contact with other inmates or the outside world. Most prisoners subject to those restrictions serve their time at the so-called Supermax prison in Florence, Colorado. But the federal government's death row is at the prison in Terre Haute, Indiana, and the judge ordered him executed under the prevailing laws of Indiana.

Tsarnaev's case is likely to result in years of appeals, and no one can say with any certainty when he might be executed, if ever.

Robert Dunham, executive director of the Death Penalty Information Center, said he expects Tsarnaev's appeals to last a decade or more.

"One can't predict the likelihood that any given death sentence will be carried out," Dunham said. "Statistically, in both federal and state death penalty cases, it is more likely that a sentence will be overturned than that it will be carried out."

He added it was difficult to predict the success of any appeal because of the secretive way in which the case was argued.

"Many of the pretrial proceedings took place under seal, and some of the sidebars were under seal," Dunham said. "We will not know until those transcripts are unsealed what other issues may be present in the case."

Just 75 people have received federal death sentences since modern death penalty laws went into force in 1988, according to the Death Penalty Information Center's website.

Only three federal death sentences have been carried out:

Timothy McVeigh was executed in June 2001 for the 1995 bombing of the Oklahoma City federal building.

• Juan Raul Garza, a marijuana trafficker convicted of murdering three drug dealers in Texas, also was executed in June 2001.