WASHINGTON, June 24 (Xinhua) -- U.S. federal judge in Boston on Wednesday officially sentenced 21-year-old Dzhokhar Tsarnaev to death for his role in the 2013 Boston Marathon bombing attacks and the following killing of a police officer when on the run.

Before the federal judge read out the ruling in court, Tsarnaev broke a two-year silence, apologizing to victims for the bombings he and his brother Tamerlan Tsarnaev unleashed at the finish line of the marathon in April 2013, which killed three people and injured 264 others.

"I'd like to now apologize to the victims and survivors," Tsarnaev spoke at the Boston courtroom before the federal judge announced the ruling of sentencing Dzhokhar Tsarnaev to death.

"I am sorry for the lives I have taken, for the damage that I have done, the irreparable damage," he added in low voice.

Tsarnaev was 19 when he carried out the twin deadly bombings with Tamerlan Tsarnaev at the crowded finishing line of the Boston Marathon, a signature event, more than two years ago. Tamerlan Tsarnaev was killed in a confrontation with the police during manhunt soon after the bombings.

Since the trial on Dzhokhar Tsarnaev began on March 4, prosecutors have called 92 witnesses while the defense summoned only four.

Though Tsarnaev pleaded not guilty, from the beginning of the trial, his defense team focused on trying hard to convince the jurors that he was under the influence of his dominant older brother, a defense tactic that was designed not to win the trial, but to avoid the death penalty.

The U.S. federal jury in Boston -- a panel of seven women and five men -- convicted Tsarnaev in April of all 30 charges against him, including 17 counts that carried the death penalty.