Marathon survivor Lynn Julian (center) speaks to the media, joined by Henry Borgard (left) and Scott Weisberg, after convicted Boston Marathon bomber Dzhokhar Tsarnaev was formally sentenced at the federal courthouse in Boston, Massachusetts June 24, 2015. Tsarnaev on Wednesday apologised for the deadly 2013 attack at a hearing before a US judge formally sentenced him to death for killing four people and injuring 264 in the bombing and its aftermath. [Photo/Agencies]

WASHINGTON - US 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.