The lawyers filed a motion in U.S. District Court asking for removal of Tsarnaev's alleged "betrayal" of the United States as an aggravating factor. They said the factor has not been used in any of the 493 federal death penalty cases since 1988.
Tsarnaev, now 20, allegedly worked with his brother to set off two bombs at the finish line of last year's Boston Marathon, killing three people and injuring many more. His older brother, Tamerlan, 26, was killed in a confrontation with police a few days later.
The brothers came to the United States as children with their parents, who are ethnic Chechens.
Defining "betrayal of the United States," prosecutors said: “Dzhokhar Tsarnaev received asylum from the United States; obtained citizenship and enjoyed the freedoms of a United States citizen; and then betrayed his allegiance to the United States by killing and maiming people in the United States.”
The defense lawyers described the move as unprecedented.
“But in not one of the 492 cases before Mr. Tsarnaev’s has the government cited the fact of a defendant’s American citizenship, the way he became a citizen, any aspect of his immigration history or his enjoyment of the freedoms of an American citizen as a reason to sentence him to death,” the motion said.
The federal government has held three executions since the U.S. Supreme Court ruled in 1976 that capital punishment is constitutional and now has 59 people on death row. The U.S. military has executed no one in the modern era and has six prisoners under death sentences.