US Attorney Carmen Ortiz (C) answers a reporter's question after announcing federal indictments against accused Boston Marathon bomber Dzhokhar Tsarnaev in Boston, Massachusetts June 27, 2013. [Photo/Agencies]

EXECUTION LOOMS?

Ortiz said US Attorney General Eric Holder would make the final decision on whether to seek the death penalty. Legal experts said that while the large scale of the attack could motivate the government to seek the death penalty, his defense could argue that he did not fully understand his actions.

"There will be claims about his youth, about his role, the theory that it was his brother that was pulling all the strings and that this guy was a secondary mover," said Richard Broughton, an assistant professor at the University of Detroit Mercy School of Law and a former federal prosecutor.

"We haven't really had a case like this," said Karen Greenberg, director of the center on national security at Fordham Law School in New York. "Because of the lethality of this attack, it really is different from other terrorism prosecutions we've seen for a long time."

Since the World Trade Center attack in 2001, most such prosecutions have focused on failed plots, such as shoe bomber Richard Reid, who tried to blow up a plane over the Atlantic in December 2001 and is now serving a life sentence. In 2006 a jury rejected the death penalty for Zacarias Moussaoui, convicted as one of the conspirators behind the September 11, 2001 attacks.

In November, 2012, a federal judge in Massachusetts sentenced Rezwan Ferdaus to 17 years, including a decade of supervised release, after he pleaded guilty to charges stemming from a plot to a plot to fly remote-controlled planes into the US Capitol building and the Pentagon. He was arrested in a sting operation after undercover agents provided him with inert explosives that he told them he planned to use.

Tamerlan Tsarnaev had been on a US government database of potential terrorism suspects and Russia has twice warned the United States that he might be an Islamic militant, according to US security officials.

A congressional hearing after the bombing focused on whether the FBI paid sufficient heed to Moscow, which has been in bitter conflict with Islamic militants in Chechnya and other parts of the volatile northern Caucasus region.

The Tsarnaev brothers' ethnic homeland of Chechnya, a mainly Muslim province that saw centuries of war and repression, no longer threatens to secede from Russia. But it has become a breeding ground for a form of militant Islam whose adherents have spread violence to other parts of Russia, and may have inspired the radicalization of the Boston bombers.