WASHINGTON, June 24 -- 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 court judge read out the ruling, 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 260 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.