THE surviving suspect in the Boston Marathon bombings has been charged with conspiring to use a weapon of mass destruction and could face a death sentence.

In a statement, US Attorney General Eric Holder detailed the charge against 19-year-old Dzhokhar Tsarnaev, who is in a serious but stable condition.

Tsarnaev made his first appearance before a magistrate judge in Beth Israel hospital, according to Gary Wente, circuit executive of the 1st US Circuit Court of Appeals.

Officials said Tsarnaev and his older brother and suspected co-conspirator, Tamerlan, set off the twin explosions at the marathon on Monday last week, killing three people and injuring more than 180 others.

The White House said Tsarnaev will not be tried as an enemy combatant in a military tribunal because he is a naturalised US citizen. The brothers were born in southern Russia.

White House spokesman Jay Carney said he will be prosecuted in the federal court system.

Tsarnaev last night remained unable to speak, with a gunshot wound to the throat.

He is expected to face separate state charges in the fatal shooting of a university police officer.

Yesterday, the city marked the traumatic week with mournful silence and a return to its bustling commute afterwards.

Massachusetts Governor Deval Patrick asked residents to observe a moment of silence at 2.50pm local time, a week to the day when the first of the two bombs exploded near the finish line.

Bells rang across the city and state after the minute-long tribute to the victims.

The White House said President Barack Obama observed the moment of silence privately.

Tsarnaev was discovered on Friday night hiding in a boat covered by a tarpaulin in suburban Watertown.

His brother was earlier killed during a shoot-out with police in an escape bid.

The motive of the two ethnic Chechen brothers remains unclear.

A private funeral took place yesterday for Krystle Campbell, a 29-year-old restaurant worker killed in the blasts, and a memorial service was held last night at Boston University for 23-year-old Lu Lingzi, a graduate student from China.

The parents of Tamerlan Tsarnaev insisted he went to Dagestan and Chechnya last year to visit relatives and had nothing to do with the militants operating in the volatile part of Russia.

His father said he slept much of the time.

A lawyer for Tamerlan Tsarnaev's wife said federal authorities have asked to speak with her.

Attorney Amato DeLuca said Katherine Russell Tsarnaev did not suspect her husband of anything, and there was no reason for her to have suspected him.

He said she had been working 70 to 80 hours, seven days a week, as a home healthcare aide.

While she was at work, her husband cared for their toddler daughter, he said.