Federal prosecutors were preparing charges Sunday against the surviving suspect in the Boston Marathon bombings even as authorities said they believed he and his brother were allegedly preparing to carry out more attacks when their plans were disrupted.
Authorities have not said publicly what charges will be filed against Dzhokhar 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.
Tsarnaev, 19, remains in serious but stable condition with a gunshot wound to the side of the 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 an earlier shootout with police that left his older brother -- the other man wanted in the bombings -- dead, said the source, who spoke on condition of anonymity.
"He's not in a condition to be interrogated at this time," Boston Police Commissioner Ed Davis told reporters at a news conference on Sunday afternoon.
However, another senior federal official who has been briefed on the investigation told CNN's Fran Townsend that Tsarnaev has communicated in writing with officials several times.
Even as prosecutors worked to put together a case against Tsarnaev, Davis said he believed the brothers were planning another attack before a 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 fire power -- 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 with knowledge of the investigation told CNN. The official, who was not authorized to publicly discuss the case, said authorities are trying to trace the guns.
Tsarnaev was captured Friday night, days after he and his brother allegedly planted two bombs near the finish line of the Boston Marathon. The blasts killed three people and wounded more than 170 others.
Authorities virtually shut down Boston and some of its suburbs as part of a manhunt after authorities say the brothers went on a rampage late Thursday and early Friday. The brothers allegedly hurled explosives at police after killing Massachusetts Institute of Technology police Officer Sean Collier and hijacking a car.
What's next for the suspect?
There are questions about whether Tsarnaev will ever be able to talk given the nature of his wound, Sen. Dan Coats, R-Indiana, said Sunday on ABC's "This Week."
"It 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," said Coats, a member of the Senate Armed Services Committee.
The government has invoked the public safety exception in the case, a designation that allows investigators to question Tsarnaev without reading him his Miranda rights and without a lawyer present, said another Justice Department official, also speaking on condition of anonymity for the same reason.
Secretary of Defense Chuck Hagel told reporters on a flight to Israel that he has not seen any intelligence that linked the brothers to any terrorist organization, but it was still early in the investigation.
But Davis told CNN 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," he said Sunday.
The sentiment was echoed by Watertown Police Chief Edward Deveau.
"From what I know right now, these two acted together and alone," he said. "I think we have to be ever vigilant, and we're learning as we go along, but as far as this little cell -- this little group -- I think we got our guys."
Hints of a radical?
While investigators piece together the actions of the brothers in the months and days before the marathon bombings, there appear to be hints that 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 Saturday 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 a strong believer and that he had changed drastically since 2010.