Boston bomb suspect Dzhokhar Tsarnaev could face the death penalty after being charged on Monday for his alleged role in the attacks which left three people dead and 200 wounded.

Tsarnaev, 19, was arraigned in his hospital bed on charges of using and conspiring to use a weapon of mass destruction, and malicious destruction of property by means of deadly explosives, the US Department of Justice said.

The charges came as America marked the one-week anniversary of the twin marathon bombings with a moment of silence observed across the nation, from the streets of Boston to the White House to the stock exchange in New York.

If Tsarnaev, a naturalized US citizen of Chechen descent, is convicted of the federal charges, he could be sentenced to jail time -- or even death.

"We've once again shown that those who target innocent Americans and attempt to terrorize our cities will not escape from justice," said US Attorney General Eric Holder. A first court hearing was set for May 30.

The unsealing of the federal charges against Tsarnaev, who suffered a gunshot wound to the throat 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," Carney said, after some Republicans had said Tsarnaev should have the same status as the "war on terror" detainees held in Guantanamo Bay.

"The system has repeatedly proven that it can successfully handle the threat we continue to face," Carney said, adding that US law forbids trying citizens in military courts.

Tsarnaev was caught after a massive manhunt that virtually shut down Boston and its suburbs on Friday. His brother and alleged accomplice Tamerlan, 26, had been killed in a chaotic overnight shootout with police.

It was still unclear if the younger Tsarnaev's severe throat injury came at the hands of police or was self-inflicted in a suicide attempt. He is reportedly unable to speak but is communicating with authorities in writing.

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 calmly walking away from the scene after the first bomb was detonated, according to the affidavit.

As counterterrorism agents trained in interrogating "high-value" detainees waited at Beth Israel Deaconess Medical Center to learn more from the teen, Bostonians attempted to put the traumatic week behind them.

The city honored the victims of the blasts with a moment of silence at 2:50 pm (1850 GMT) that was also observed in Washington, by President Barack Obama and lawmakers, and in New York, at the stock exchange.

Hundreds gathered outside the security cordon set up near the blast sites at the marathon finish line on Boylston Street to honor the dead and wounded. Some prayed, others left flowers. Church bells rang out across the city.

"I am here for a friend seriously injured and for all the victims," teacher John Abbott, who was unable to finish last week's race, told AFP.

"Boston on the surface is a very cold city, but when you get below the surface, this is what you get. People are all business here so it is impressive to see people stop for a minute and show what they really feel."

FBI investigators are still hoping to get answers from Tsarnaev about the brothers' possible motive, and learn whether other attacks were in the works.

Boston Police Commissioner Ed Davis said Sunday 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 radicalized or trained there.

Tamerlan had begun posting militant videos on social media sites in recent years. Both Russian regions are home to Islamist and separatist groups, but rebels in Dagestan have denied any link to the Boston bombings.

The Tsarnaev family came to the United States from the former Soviet state of Kyrgyzstan around 2002. Dzhokhar became a US citizen in 2012, while Tamerlan's application was reportedly held up due to the FBI questioning.