Boston Marathon bomber Dzhokhar Tsarnaev on Thursday was moved to a U.S. penitentiary in Florence, Colorado, home to the so-called "Supermax" unit that houses high-risk prisoners, a spokesman for the Bureau of Prisons said.

Tsarnaev, 21, was moved to the facility the day after a federal judge in Boston formally sentenced him to death by lethal injection for killing four people and injuring 264 in the April 15, 2013, bombing and its aftermath.

Bureau of Prisons spokesman Edmond Ross confirmed the move.

Tsarnaev had previously been held at a prison facility in Devens, Massachusetts, west of Boston.

At Wednesday's hearing, after listening to two dozen survivors of the attack and relatives of victims, Tsarnaev addressed the court for the first time.

"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," he told the court.

The prisoners at the super-maximum security penitentiary in Florence have no cellmates, and recreation is solitary, according to a former federal prison warden who testified during Tsarnaev's trial.

Inmates at the facility include Oklahoma City bomber accomplice Terry Nichols, underwear bomber Umar Farouk Abdulmutallab and Unabomber Ted Kaczynski.