U.S. prosecutors on Thursday announced they will seek the death penalty against 20-year-old Dzhokhar Tsarnaev in the Boston Marathon bombing, instantly raising the stakes in what could be one of the most wrenching trials the city has ever seen.

Attorney General Eric Holder's decision to press for Tsarnaev's execution was widely expected. The twin blasts killed three people and wounded more than 260 others, and 17 of the 30 federal charges against him — including using a weapon of mass destruction to kill — carry the possibility of the death penalty.

"The nature of the conduct at issue and the resultant harm compel this decision," Holder said in a statement.

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Tsarnaev has pleaded not guilty. No trial date has been set.

In a notice of intent filed in court, federal prosecutors in Boston listed factors they contend justify a sentence of death, including his "betrayal" of the U.S., where he had lived since moving from Russia about a decade ago.

"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," read the notice filed by U.S. Attorney Carmen Ortiz.

'It shows people that if you are going to terrorize our country, you are going to pay with your life.' - Marc Fucarile, Boston Marathon victim

Prosecutors also cited Tsarnaev's "lack of remorse" and allegations that he killed an MIT police officer as well as an eight-year-old boy, a "particularly vulnerable" victim because of his age. They also said Tsarnaev committed the killings after "substantial planning and premeditation."

Tsarnaev's lawyers had no immediate comment.

In an interview with ABC, Tsarnaev's mother, Zubeidat, who lives in Russia, said: "How can I feel about this? I feel nothing. I can tell you one thing, that I love my son. I will always feel proud of him. And I keep loving him."

Prosecutors allege Tsarnaev, then 19, and his 26-year-old brother, ethnic Chechens from Russia, built and planted two pressure-cooker bombs near the finish line of the marathon in April to retaliate against the U.S. for its military action in Muslim countries.

The older brother, Tamerlan Tsarnaev, died in a shootout with police during a getaway attempt days after the bombing. Dzhokhar Tsarnaev was wounded but escaped and was later found hiding in a boat parked in a yard in a Boston suburb.

Authorities said he scrawled inside the boat such things as "The US Government is killing our innocent civilians" and "We Muslims are one body, you hurt one you hurt us all."

16 people lost limbs

Killed in the bombings were: Martin Richard, 8, of Boston; Krystle Campbell, 29, of Medford; and Lu Lingzi, 23, a Boston University graduate student from China. At least 16 others lost limbs.

Tsarnaev is also charged in the slaying of the MIT police officer and the carjacking of a motorist during the brothers' getaway attempt.

Campbell's grandmother, Lillian Campbell, said she doesn't think Tsarnaev should live but isn't sure she supports the death penalty, even though she fears he will "end up living like a king" in prison.

"I don't know, because it's not going to bring her back," she said. "I don't even like to discuss it because it makes me so upset. She was my granddaughter and I miss her so much."

"I think it's the right decision to go after the death penalty," said Marc Fucarile, who lost his right leg above the knee
and suffered other severe injuries in the bombing. "It shows people that if you are going to terrorize our country, you are going to pay with your life."

Amato DeLuca, a lawyer for Tamerlan Tsarnaev's widow, said: "Whatever he's alleged to have done, presumably he can pay for it with his life. Putting this boy to death doesn't make any sense to me."

Tsarnaev's case has attracted a high-profile defence team, including Judy Clarke, one of the nation's foremost death penalty specialists. The San Diego attorney negotiated plea agreements that saved the lives of such clients as the Unabomber and Olympic Park bomber Eric Rudolph.

Legal experts have said that court filings suggest the defence will try to save Tsarnaev's life by arguing that he fell under the evil influence of his older brother.

In a statement, Massachusetts Gov. Deval Patrick said: "One way or another, based on the evidence, Tsarnaev will die in prison." He did not say whether he supports the decision to seek the death penalty. Patrick opposes capital punishment.

"In each milestone of this case — today's announcement, the trial and every other significant step in the justice process — the people hurt by the marathon bombings and the rest of us so shocked by it will relive that tragedy," he said.

Massachusetts generally against death penalty

Massachusetts abolished its state death penalty in 1984, and repeated attempts to reinstate it have failed in the Legislature. A Boston Globe poll conducted in September found that 57 per cent of those questioned favoured a life sentence for Tsarnaev, while 33 per cent supported the death penalty for him.

Two other federal death penalty cases have been brought in Massachusetts. A former veterans hospital nurse who killed four patients by overdosing them was spared the death penalty by a jury. A man accused in the carjack killings of two Massachusetts men was sentenced to death in 2003, but the punishment was overturned and he is awaiting a new penalty trial.

Jurors for federal cases tried in Boston are drawn from the Boston metropolitan and eastern Massachusetts — a politically liberal region, but also the part of the state most directly affected by the tragedy.​

The American Civil Liberties Union of Massachusetts, which opposes the death penalty in all cases, objected to Holder's decision.

"After the horrible marathon attack, this community rallied around the slogan `Boston Strong,"' said Carol Rose, executive director of the ACLU. "Even — and especially in a case like this — that means not letting terrorists or anyone else shake us from staying true to our values."

Since the federal death penalty was reinstated in 1988, 70 death sentences have been imposed, but only three people have been executed, including Oklahoma City bomber Timothy McVeigh in 2001.

The last federal execution was in 2003, when Gulf War veteran Louis Jones Jr. was put to death for kidnapping 19-year-old Army Pvt. Tracie McBride from a Texas military base, raping her and beating her to death with a tire iron.

Massachusetts abolished its state death penalty in 1984, and repeated efforts to reinstate it have failed.