By David Ingram and Richard Valdmanis

WASHINGTON/BOSTON (Reuters) - The United States will seek the death penalty for accused bomber Dzhokhar Tsarnaev, who is charged with planting homemade explosives devices that killed three people and wounded 264 at the Boston Marathon last year, the government's chief prosecutor said on Thursday.

U.S. Attorney General Eric Holder said in a statement that he was authorizing trial prosecutors to seek the death penalty against Tsarnaev, who is charged with committing one of the largest attacks on U.S. soil since September 11, 2001.

"The nature of the conduct at issue and the resultant harm compel this decision," Holder said. Holder had faced a Friday deadline for deciding whether to seek the death penalty as part of Tsarnaev's upcoming trial in Boston.

Government prosecutors said in a filing with the U.S. District Court in Boston that reasons for Holder's decision included that the killings were premeditated, cruel and that Tsarnaev had shown a lack of remorse.

"One way or another, based on the evidence, Tsarnaev will die in prison," Massachusetts Governor Deval Patrick said. A trial date has not yet been set for Tsarnaev, who has pleaded not guilty to the charges.

Boston Mayor Martin Walsh reacted to Holder's announcement by saying he supported "the process that brought him to this decision," adding that his thoughts were with the victims of the bombing and their families.

"We stand together as one Boston in the face of evil and hatred," he said.

UNUSUAL CRIME

Prosecutors say that 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 April 15, 2013, killing three people - including an 8-year-old boy. The blast also wounded 264 others, many of whom lost limbs.

Three nights later, the ethnic Chechen brothers killed a university police officer and later engaged in a shootout with police that left Tamerlan dead, prosecutors say. Dzhokhar was later found hiding in a boat in which he scrawled several phrases, including "we Muslims are one body, you hurt one you hurt us all", according to prosecutors.

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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 a 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."

Tsarnaev's attorneys have argued against a possible death sentence, in part because they claim he 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 defense, including seeking to rush the start of trial and not sharing important evidence.

Tsarnaev's defense attorney Miriam Conrad declined to comment on Holder's decision on Thursday.

Holder has said that 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 authorized prosecutors to seek the death penalty in 36 cases, according to the Justice Department.

Legal experts said that if Tsarnaev is convicted, the jury would ultimately decide whether to apply the death penalty or a lesser sentence like life in prison.

CRITICISM FROM ACLU

Holder's decision immediately 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.

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

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

Only three people have been executed as the result of a federal capital case since 1988, when the United States reinstated the federal death penalty, including Oklahoma City federal building bomber Timothy McVeigh in 2001.

"This development will ensure that many of the victims feel like they are getting justice, but it will also extend and complicate the prosecution and dramatically increase the cost to taxpayers," said Steve Huggard, a former federal prosecutor who is now Boston-based partner at Edwards Wildman.

"This case now could easily last a decade."

The blasts killed 8-year-old Martin Richard as well as Krystle Campbell, 29, and Chinese national Lu Lingzi, 23. Tsarnaev is also accused in the shooting death of Sean Collier, 27, the university police officer.

A spokesman for Richard's family said the family did not want to comment. Efforts to reach the families of the other victims were not immediately successful.

A trial date for Tsarnaev has not yet been set.

(Additional reporting by Ross Kerber, Tim McLaughlin and Joan Biskupic; editing by Howard Goller, James Dalgleish, G Crosse, Bernard Orr, Tom Brown and Cynthia Osterman)