(CNN) -- Unable to speak, the surviving suspect in the Boston Marathon bombings has been communicating with authorities in writing, a senior federal official briefed on the investigation told CNN Monday.
Investigators have been questioning Dzhokhar Tsarnaev since Sunday, according to a second source -- a senior law enforcement official.
Neither source would say what, if anything, Tsarnaev has been telling investigators about his alleged role in the bombing that killed three and wounded more than 170 a week ago Monday.
While Tsarnaev, 19, has not yet been charged with a crime, investigators believe he and his brother, Tamerlan Tsarnaev, were behind the attacks.
Tamerlan Tsarnaev, 26, died early Friday at a hospital after a shootout with police.
While authorities say Bostonians can rest easier now that the younger Tsarnaev is in custody, nagging questions hinder any total sense of security: Why would the assailants want to kill or maim throngs of innocent civilians, and could this happen again?
Massachusetts Gov. Deval Patrick and Boston Mayor Thomas Menino have asked Bostonians and residents in the rest of the state to observe a moment of silence at 2:50 p.m. Monday, exactly a week after twin explosions near the marathon's finish line. Bells will toll one minute later to honor the victims of the tragedy that traumatized the city.
Police chief: The carnage could have been worse
In the tumultuous days after the bombings, the Tsarnaev brothers allegedly killed a university police officer, led authorities on a harrowing chase and hurled explosives at police, authorities said. Another officer, seriously wounded in a firefight with the suspects, was recovering Monday, Boston Police Commissioner Ed Davis said.
Tamerlan Tsarnaev died after a gun battle with officers in the Boston suburb of Watertown in which more than 200 rounds were exchanged. Authorities have yet to announce the official cause of death.
According to Watertown Police Chief Edward Deveau, the elder Tsarnaev stepped out of the car he and his brother had allegedly stolen earlier that night and began walking toward officers, firing as he went.
When he ran out of ammunition, officers tackled him and tried to handcuff him, Deveau said Saturday.
But Dzhokhar Tsarnaev came barreling at them in the stolen vehicle, the chief said. The officers scrambled out of the way, and the vehicle then ran over the older brother and dragged him for a short distance.
Tamerlan Tsarnaev also had explosives on his body, officials have said.
Dzhokhar Tsarnaev was found bleeding later that night, hiding in a boat in a backyard in Watertown, Massachusetts.
Police say they believe the brothers were planning another attack before the shootout with police disrupted their plans, Davis said.
"The two suspects were armed with handguns at the scene of the shootout, and there were multiple explosive devices, including a large one that was similar to the pressure cooker device that was found on Boylston Street," Davis said on CNN's "Starting Point" Monday.
"I believe that the only reason that someone would have those in their possession was to further attack people and cause more death and destruction," he said.
Authorities believe the brothers bought bomb components locally but think that their guns came from elsewhere, another federal law enforcement official said. The official, who was not authorized to publicly discuss the case, said authorities are trying to trace the guns.
Investigators are also trying to determine whether anyone else was involved in the bombings.
But Davis, speaking Sunday to CNN's Don Lemon, said that he was confident that the brothers were "the two major actors in the violence that occurred."
"I told the people of Boston that they can rest easily, that the two people who were committing these vicious attacks are either dead or arrested, and I still believe that," the police chief said.
Clues about radicalization?
While investigators piece together the brothers' actions leading up to the marathon bombings, details have emerged suggesting the elder Tsarnaev was turning radical.
The Tsarnaev family hails from the Russian republic of Chechnya and fled the brutal wars there in the 1990s. The two brothers were born in Kyrgyzstan, authorities said.
An FBI official said agents interviewed Tamerlan Tsarnaev in 2011 at the request of the Russian government. The FBI said Russia claimed that he was a follower of radical Islam and that he had changed drastically since 2010.
But the Russian government's request was vague, a U.S. official and a law enforcement source said Sunday. The lack of specifics limited how much the FBI was able to investigate Tamerlan, the law enforcement official said.
Tamerlan Tsarnaev apparently became increasingly radical in the past three or four years, according to an analysis of his social media accounts and the recollections of family members. But so far, there has been no evidence of active association with international jihadist groups.
In August 2012, soon after returning from a visit to Russia, the elder Tsarnaev brother created a YouTube channel with links to a number of videos. Two videos under a category labeled "Terrorists" were deleted. It's not clear when or by whom.
According to The Boston Globe, Tamerlan Tsarnaev disrupted a service a Cambridge mosque in January after a speaker likened the Muslim Prophet Mohammed to Martin Luther King Jr. The congregation "shouted him out of the mosque," the Globe quoted mosque spokesman Yusufi Vali as saying.
A week after the marathon bombings, 55 people remain hospitalized, including three in critical condition, according to a CNN tally.
At least a dozen survivors have endured amputations.
The transit system officer wounded in the firefight with the Tsarnaev brothers, Richard Donohue, was improving Monday, Davis said.
"He was in grave condition when he went to the hospital, so we're very optimistic at this point in time, and our prayers are with him and his family," he said.
Dzhokhar Tsarnaev, meanwhile, remains in serious but stable condition, according to the U.S. Attorney's Office District of Massachusetts. A federal law enforcement source with knowledge of the investigation told CNN the younger brother has a gunshot wound to his neck, and he had a tube down his throat to help him breathe.
It's unclear whether Tsarnaev was wounded during his capture or in the earlier shootout with police that left his older brother dead, said the source, who spoke on condition of anonymity.
Tsarnaev had not been charged as of Monday morning.
Authorities have not publicly stated what charges will be filed against Tsarnaev, but a Justice Department official who has been briefed on the case told CNN he will face federal terrorism charges and possibly state murder charges.
While Massachusetts does not have a death penalty, prosecutors could seek the death penalty at the federal level.
Getting back to normal
It could take up to two more days before the area around the site of the explosions can reopen to the public, Davis said.
The FBI has not yet turned the scene back over to local authorities, the police chief said.
"We have to allow store owners to go in there first. It won't be open to the general public for maybe another day so the store owners can get their business back on track," Davis said. "We want to get people back in their homes as soon as possible, and we're working diligently on that right now."
Also on Monday, the one-week anniversary of the Boston attacks, thousands of runners across the country will pound the pavement in a show of unity and support for the victims and their families.
At least 80 cities are participating in a "Run for Boston in Your City" campaign called #BostonStrong, organizer Brian Kelley said.
The global campaign is "a run for those that were unable to finish, a run for those that may never run again" and "a run for us to try and make sense of the tragedy that has forever changed something we love."
CNN's Gloria Borger, Fran Townsend, Tim Lister, Paul Cruickshank, Deborah Feyerick, Jill Dougherty, Pamela Brown, Julian Cummings, Barbara Starr, Susan Candiotti and Jake Tapper contributed to this report.
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