NATIONAL NEWS (CNN) — At 2:50 p.m. Monday, bells across Boston will chime to mark a tragedy that unhinged the city.
As the country pauses to reflect on the Boston Marathon attacks exactly one week ago, the lone surviving suspect remains hospitalized with a tube down his throat, unable to verbalize what went through his mind the day a pair of bombs killed three people and wounded more than 170 others.
While authorities say Bostonians can rest easier now that Dzhokhar Tsarnaev is in custody, a nagging question hinders any complete sense of security: Why would the assailants want to kill and maim throngs of innocent civilians?
Police chief: The carnage could have been worse
In the tumultuous days since the bombings, Tsarnaev and his older brother Tamerlan killed a university police officer, led authorities on a harrowing chase and hurled explosives at police, authorities said.
Tamerlan Tsarnaev died after an epic gun battle with officers in which more than 200 rounds were exchanged. Dzhokhar Tsarnaev was found less than a day later, hiding in a boat and bleeding in a Watertown man's backyard.
Boston Police Commissioner Ed Davis said he believed the brothers were planning another attack before the shootout with police disrupted their plans.
"We have reason to believe, based upon the evidence that was found at the scene -- the explosions, the explosive ordnance that was unexploded and the firepower -- that they were going to attack other individuals," Davis said Sunday on CBS News' "Face the Nation."
He did not say whether investigators had identified a specific target.
Authorities believe the brothers bought bomb components locally but 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 behind the bombings.
But Davis told CNN's Don Lemon 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 Sunday.
Clues of radicalization?
While investigators piece together the brothers' actions in the months and days before the marathon bombings, details have emerged suggesting the elder Tsarnaev was becoming 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 he was a follower of radical Islam and that he had changed drastically since 2010.
But a U.S. official and a law enforcement source said Sunday the Russian government's request was vague. 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.
A CNN analysis of the YouTube channel determined one deleted video featured a militant named Abu Dujana, whose real name was Gadzhimurad Dolgatov. CNN has located a video clip of the footage in question.
Russian security services killed Dolgatov in December during an assault on an apartment in the Russian Caucasus republic of Dagestan. Dolgatov led a small group in Dagestan that had links to the main Islamist militant group in the region, Imarat Kavkaz.
Separately, a U.S. intelligence source told CNN that investigators are looking into whether Tsarnaev had any connections with the group, known in English as Caucasus Emirates. The source says Tsarnaev had several computer links to the group in his social media activities, and investigators are looking into the possibility that he received "operational plans" from this group.
Rebels who call themselves Mujahideen of the Caucasus Emirate Province of Dagestan issued a statement Sunday that appeared to distance the group from the Boston bombings, saying they are not fighting the United States.
"We are at war with Russia," it said. The statement also said that children are never targets of the group.
A week after the marathon bombings, 55 remain hospitalized, including three in critical condition, according to a CNN tally.
At least a dozen survivors have endured amputations.
Tsarnaev, meanwhile, remains in serious but stable condition with a gunshot wound on his neck, a federal law enforcement source with knowledge of the investigation told CNN on Sunday.
It is 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.
"He's not in a condition to be interrogated at this time," Davis told reporters Sunday afternoon.
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.
A run to remember
On 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 #BostonStrong, a "Run for Boston in Your City" campaign, organizer Brian Kelley said.
The global campaign is "a run for us to unite and show our strength, 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 Tim Lister, Paul Cruickshank, Deborah Feyerick, Jill Dougherty, Pamela Brown, Julian Cummings, Barbara Starr, Ann O'Neill, Susan Candiotti, Jake Tapper, Lindy Royce and Drew Griffin contributed to this report.