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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.

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.