By Scott Malone and Richard Valdmanis
BOSTON (Reuters) - Boston Marathon bomber Dzhokhar Tsarnaev on Wednesday apologized for the deadly 2013 attack at a hearing before a U.S. judge formally sentenced him to death for killing four people and injuring 264 in the bombing and its aftermath.
The 21-year-old ethnic Chechen, who had not testified during his trial, referred to Allah and admitted that he and his now-dead older brother carried out one of the highest-profile attacks on U.S. soil, in a courtroom packed with survivors of the April 15, 2013 bombing.
"I am sorry for the lives I have taken, for the suffering that I have caused you, for the damage I have done, irreparable damage," said Tsarnaev, who had sat in silence, his head cast down as two survivors and family members of victims described the attacks' heavy toll on their lives.
"In case there is any doubt, I am guilty of this attack, along with my brother," Tsarnaev said, standing at the defense table.
Tsarnaev had been found guilty killing three people and injuring 264 in the bombing near the finish line of the world-renowned race, as well as fatally shooting a police officer three days later. The same federal jury that convicted him in April voted for death by lethal injection in May.
As he handed down that sentence, U.S. District Judge George O'Toole condemned Tsarnaev for falling under the spell of militant Islamists, including American-born al Qaeda figure Anwar al Awlaki, who was killed in a 2011 drone strike.
"It is tragic ... that you succumbed to their demonic siren song," O'Toole said. "As long as your name is mentioned, what will be remembered is the evil you've done."
Before the judge pronounced the sentence, Rebekah Gregory, who lost her left leg on that blood-soaked April day, addressed Tsarnaev directly.
"Terrorists like you do two things in this world. One, they create mass destruction, but the second is quite interesting," Gregory said. "Because do you know what mass destruction really does? It brings people together. We are Boston strong and we are America strong, and choosing to mess with us was a terrible idea.
Tsarnaev's trial brought back some of Boston's darkest living memories. Jurors saw videos of the bombs' blinding flashes and the chaotic aftermath as emergency workers and spectators rushed to aid the wounded, many of whom lost legs.
Three people died in the bombing: Martin Richard, 8, Chinese exchange student Lingzi Lu, 26, and restaurant manager Krystle Campbell, 29. Three days later, Tsarnaev and his 26-year-old brother, Tamerlan, shot dead Massachusetts Institute of Technology police officer Sean Collier, 26.
Tamerlan Tsarnaev died following a gunfight with police that ended when Dzhokhar ran him over with a car.
During the trial, federal prosecutors described the brothers as adherents of al Qaeda's militant Islamist ideology who wanted to "punish America" with the attack on the world-renowned marathon.
Tsarnaev's attorneys admitted their client had played a role in the attack but tried to portray him as the junior partner in a scheme hatched and driven by his older brother. The Tsarnaev family came to the United States from Russia a decade before the attack.
Boston has been on high alert since the attack and its aftermath. Police were out in force around the waterfront courthouse all day Wednesday.
At midday, Boston Police arrested a man outside the courthouse, and said he had a meat cleaver in his possession.
"In today's threat environment, you can't overlook anything," Vincent Lisi, the Federal Bureau of Investigation's top agent in Boston, told reporters. He added that members of a joint terrorism task force were interviewing the man.
Even after the sentencing, the legal wrangling over Tsarnaev's fate could play out for years, if not decades. Just three of the 74 people sentenced to death in the United States for federal crimes since 1998 have been executed.
Krystle Campbell's mother, Patricia, called Tsarnaev's actions "despicable."
"You went down the wrong road," Campbell said. "I know life is hard, but the choices you made were despicable and what you did to my daughter was disgusting."
Tsarnaev asked forgiveness for himself and his dead brother.
"I ask Allah to have mercy upon me, my brother and my family," Tsarnaev said. "I ask Allah to bestow his mercy upon those who are here today."
The government's chief prosecutor on the case, William Weinreb, said he was unimpressed by Tsarnaev's apology.
"He did this for political reasons. This was a politically motivated act," Weinreb said. "At no point during his statement did he ever renounce the motives for which he carried out this act. He never renounced terrorism."
(Reporting by Scott Malone; Editing by Lisa Von Ahn and Jonathan Oatis)