Boston Marathon bombing suspect Dzhokhar Tsarnaev was charged in his hospital room Monday with using a weapon of mass destruction to kill — a crime that carries a possible death sentence.

Tsarnaev, 19, was accused by federal prosecutors of conspiring with his older brother to set off the two pressure-cooker bombs that sprayed shrapnel into the crowd at the finish line last Monday, killing three people and wounding more than 180.

The criminal complaint containing the charges shed no light on the motive for the attack.

Holly Holland of St. Louis hugs her daughter Katie Holland while visiting a makeshift memorial in Boston on Monday. (Steven Senne/Associated Press)

Tsarnaev was listed in serious but stable condition at Beth Israel Deaconess Medical Center, unable to speak because of a gunshot wound to the throat. His brother, Tamerlan, 26, died last week in a fierce gun battle with police.

In outlining the evidence against him in court papers, the FBI said Tsarnaev was seen on surveillance cameras putting a knapsack down on the ground near the site of the second blast and then manipulating a cellphone and lifting it to his ear.

Tsarnaev looked calm after 1st explosion, court papers say

After the first explosion went off about a block down the street and spread fear through the crowd, Tsarnaev — unlike nearly everyone around him — looked calm and quickly walked away, the FBI said. Just 10 seconds or so later, the second blast occurred where he left the knapsack, the FBI said.

The FBI did not make it clear whether authorities believe he used his cellphone to detonate one or both of the bombs or whether he was talking to someone.

The court papers also said that during the long night of crime Thursday and Friday that led to the older brother's death and the younger one's capture, one of the Tsarnaev brothers told a carjacking victim: "Did you hear about the Boston explosion? I did that."

Katherine Russell Tsarnaev, wife of killed Boston Marathon bombing suspect Tamerlan Tsarnaev, exits a car at the home of her parents in North Kingstown, R.I. on Sunday. Federal authorities have asked to speak with her, and her lawyer said he is discussing with them how to proceed. (Katie Zezima/Associated Press)

The brothers are ethnic Chechens from Russia who have lived in the U.S. for about a decade. Investigators are focusing on a trip the older brother made last year to Chechnya and Dagestan, in a region of Russia that has become a hotbed of separatist politics and Islamic extremism.

Tsarnaev was charged with using and conspiring to use a weapon of mass destruction against persons and property, resulting in death. He is also likely to face state charges in connection with the shooting death of an MIT police officer.

The Obama administration said it had no choice but to prosecute Tsarnaev in the federal court system. Some politicians had suggested he be tried as an enemy combatant in front of a military tribunal, where defendants are denied some of the usual U.S. constitutional protections.

Memorials A funeral mass was held starting at 11 a.m. ET Monday for 29-year-old Krystie Campbell in her hometown of Medford, Mass. A memorial service was planned Monday evening at Boston University to remember 23-year-old graduate student Lingzi Lu. On Sunday, a memorial was held in Dorchester, Mass., for eight-year-old Martin Richard.

But Tsarnaev is a naturalized U.S. citizen, and under U.S. law, American citizens cannot be tried by military tribunals, White House spokesman Jay Carney said. Carney said that since the Sept. 11 attacks, the federal court system has been used to convict and incarcerate hundreds of terrorists.

In its criminal complaint, the FBI said it searched Tsarnaev's dorm room at the University of Massachusetts-Dartmouth on Sunday and found BBs as well as a white hat and dark jacket that look like those worn by one of the suspected bombers in the surveillance photos the FBI released a few days after the attack.

Seven days after the bombings, meanwhile, Boston was bustling Monday, with runners hitting the pavement, children walking to school and enough cars clogging the streets to make the morning commute feel almost back to normal.

Residents paused in the afternoon to observe a moment of silence at 2:50 p.m. ET, the time of the first blast. Church bells tolled across the city and state in tribute to the victims.

Also, hundreds of family and friends packed a church in Medford for the funeral of bombing victim Krystle Campbell, a 29-year-old restaurant worker. A memorial service was scheduled for Monday night at Boston University for 23-year-old Lingzi Lu, a graduate student from China.

In Canada, runners in Ottawa marched from Parliament Hill to the American Embassy in memory of the victims.The one-kilometre trek began after speeches from Sharon Johnston, wife of Gov. Gen. David Johnston, and Phil Marsh, a manager of Ottawa-area Running Room stores.

Dzhokhar Tsarnaev, left, a suspect in the Boston Marathon bombings, was captured Friday night. His brother, Tamerlan Tsarnaev, a second suspect, died during a shootout with police earlier in the day. (Robin Young/The Lowell Sun/AP)

Fifty-one victims remained hospitalized Monday, three of them in critical condition.

At the Snowden International School on Newbury Street, a high school set just a block from the bombing site, jittery parents dropped off children as teachers — some of whom had run in the race — greeted each other with hugs.

Carlotta Martin of Boston said that leaving her kids at school has been the hardest part of getting back to normal.

"We're right in the middle of things," Martin said outside the school as her children, 17-year-old twins and a 15-year-old, walked in, glancing at the police barricades near the school's front door.

"I'm nervous. Hopefully, this stuff is over," she continued. "I told my daughter to text me so I know everything's OK."

Tsarnaev was captured Friday night after an intense all-day manhunt that brought the Boston area to a near-standstill. He was cornered and seized, wounded and bloody, after he was discovered hiding in a tarp-covered boat in a Watertown backyard.

He had apparent gunshot wounds to the head, neck, legs and hand, the FBI said in court papers.

Sen. Dan Coats of Indiana, a member of the Senate Intelligence Committee, said Sunday that Tsarnaev's throat wound raised questions about when he will be able to talk again, if ever. It was not clear whether the wound was inflicted by police or was self-inflicted.

The wound "doesn't mean he can't communicate, but right now I think he's in a condition where we can't get any information from him at all," Coats told ABC's This Week.