BOSTON (AFP) - Investigators on Tuesday sought a motive for the Boston Marathon bombings after the surviving suspect was charged for his alleged role in the worst attacks in the United States since September 11, 2001.
Dzhokhar Tsarnaev, 19, who remains hospitalised, spoke only one word aloud at his bedside arraignment on Monday, responding "no" when asked whether he could afford an attorney, according to a transcript of the hearing released on Monday.
But he was said to be alert as he heard the charges of using and conspiring to use a weapon of mass destruction, and malicious destruction of property by means of deadly explosives - which carry a possibly death penalty, if convicted.
Three people died and some 200 were wounded in twin blasts at the Boston Marathon on April 15, the deadliest bombings on US soil since the September 11, 2001 terror attacks involving hijacked airliners.
Counter-terrorism agents trained in interrogating "high-value" detainees are still hoping to get answers about the brothers' possible motive, and learn whether other attacks were in the works.
An unnamed US government source told CNN that "preliminary interviews with Tsarnaev indicate the two brothers fit the classification of self-radicalised jihadists," and that international groups were not involved.
The source said Tsarnaev told investigators that his older brother was the leader and "wanted to defend Islam from attack."
Tsarnaev was caught after a massive manhunt that virtually shut down Boston and its suburbs last Friday. His brother and alleged accomplice Tamerlan, 26, had been killed in a chaotic overnight shootout with police.
A clearer picture of Tsarnaev's role in the attacks emerged with the release of an affidavit from an FBI agent on Monday, which revealed the teenager had been caught on film planting the second backpack bomb.
Surveillance footage showed Tsarnaev, a naturalised US citizen of Chechen descent, calmly walking away from the scene after the first bomb was detonated, according to the affidavit.
The unsealing of the federal charges against Tsarnaev, who suffered gunshot wounds to the head, neck, legs and hand before his capture late Friday, came as White House spokesman Jay Carney said he would not be deemed an "enemy combatant."
"We will prosecute this terrorist through our civilian system of justice," Mr Carney said after some Republicans had called for Tsarnaev to have the same status as the "war on terror" detainees held in Guantanamo Bay.
At the arraignment, the federal judge said she found Tsarnaev "alert, mentally competent, and lucid," according to the transcript. A first court hearing was set for May 30.
Meanwhile, a week after the deadly attack, Bostonians honoured the victims with a moment of silence at 2.50 pm (2.50am on Tuesday, Singapore time) that was also observed in Washington by President Barack Obama and lawmakers, and in New York, at the city's stock exchange.
Hundreds gathered outside the security cordon set up near the blast sites at the marathon finish line on Boylston Street to honour the dead and wounded.
Some prayed, others left flowers. Church bells rang out across the city.
Boston Police Commissioner Ed Davis said that the brothers, who had been living legally in the United States for more than a decade, had more homemade explosive devices and appeared to have been planning more attacks.
He said federal authorities were trying to track down how and where the two suspects obtained firearms and explosive devices.
An M-4 assault rifle was recovered in the boat where Dzhokhar was captured, The New York Times reported.
The brothers also apparently used two handguns and a BB gun in the shootout with police in the Boston suburb of Watertown. They are also believed to have shot dead a campus police officer at the Massachusetts Institute of Technology.
Investigators are now probing a six-month trip made by Tamerlan in 2012 to Russia's troubled regions of Dagestan and Chechnya, and whether he was radicalised or trained there.
US Senator Lindsey Graham said the FBI and Russian intelligence may have missed warning signs and made basic errors like misspelling Tamerlan Tsarnaev's name, allowing him to travel to Russia undetected.
"I've been told by the FBI that the reason that his name did not pop up in the system was because it was misspelled," Ms Graham said.
"We don't know if he misspelled it," or if it was the fault of the airline, identified as Russian flagship Aeroflot. "We certainly missed it here."