Boston bombing suspect Dzhokhar Tsarnaev should be put to death if he is found guilty of planting bombs that killed three people and wounded 264 at the Boston Marathon last year, the US government's chief prosecutor said this evening.

US Attorney General Eric Holder said in a statement that he was authorising trial prosecutors to seek the death penalty against Mr Tsarnaev.

Mr Tsarnaev is charged with committing one of the largest attacks on US soil since 11 September 2001.

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

Tomorrow was the deadline for Mr Holder to decide whether to seek the death penalty as part of Mr Tsarnaev's upcoming trial in Boston.

The decision drew fire from the American Civil Liberties Union of Massachusetts, which pointed out the case would be prosecuted in a state that had scrapped the death penalty decades ago.

"I wish Federal officials would have respected the clear wishes of the people of Massachusetts, who were on the frontlines in this tragic event," Carol Rose, the executive director of the ACLU of Massachusetts said.

A Boston Globe survey found last year that 57% percent of Boston residents favoured life in prison for Mr Tsarnaev, if he is convicted, with 33% in favour of execution.

Prosecutors say that Mr Tsarnaev, 20, and his 26-year-old brother Tamerlan planted a pair of homemade pressure-cooker bombs at the race's crowded finish line on 15 April last year, killing three people, including an eight-year-old boy.

The blast also wounded 264 others, many of whom lost limbs.

Three nights later, prosecutors say the pair killed a university police officer and later engaged in a shootout with police that left Tamerlan dead.

Austin Sarat, Professor of Jurisprudence and Political Science at Amherst College in Massachusetts, said the nature of the case probably left the Justice Department little choice but to seek capital prosecution.

"If the harm is unusual, if the harm is dramatic, gruesome, and devastating, it is often very hard for any other factor to outweigh it," he said. "I'm not surprised by this decision."

The younger Tsarnaev pleaded not guilty to the criminal charges.

Justice Department officials said the nearly seven months since the attack was necessary to evaluate fully the circumstances of the case and to gather recommendations from prosecutors advising Mr Holder.

Mr Holder has said he is not a proponent of the death penalty because he believes its value as a deterrent is questionable, but since becoming attorney general in 2009, he has authorised prosecutors to seek the death penalty in 36 cases, according to the Justice Department.

Attorneys for Mr Tsarnaev have argued against a possible death sentence, in part because they claim Dzhokhar was following the lead of his older brother.

They have also accused the government of throwing up unfair obstacles to hinder preparation of their client's defence, including seeking to rush the start of trial and not sharing important evidence.

Mr Tsarnaev's defence attorney Miriam Conrad declined to comment on Mr Holder's decision tonight.             

The blasts killed Martin Richard, eight, 29-year-old Krystle Campbell, and 23-year-old Chinese national Lu Lingzi.

Mr Tsarnaev is also accused in the shooting death of Sean Collier, 27, the university police officer.

A trial date has not yet been set.