By Chelsea J. Carter, Holly Yan and Steve Almasy
(CNN) -- 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.
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 ordinance 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.
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, allegedly hurling explosives at police, after killing Massachusetts Institute of Technology police Officer Sean Collier and hijacking a car.
Lawmaker believes slain suspect trained in Russia
New details emerged Sunday as investigators tried to determine more about the brothers they believe were behind attacks last week.
House Homeland Security Chairman Michael McCaul said on CNN's "State of the Union" that slain suspect Tamerlan Tsarnaev received training while he was in Russia for six months in 2012.
The Texas Republican also questioned why the FBI did not take further action against Tamerlan Tsarnaev when he was investigated before his trip.
"There were concerns about this individual. And yet when he travels abroad and gets to a very dangerous part of the world, nothing seems to be done," he told CNN.
McCaul said after his final Sunday talk show appearance that he thought Tamerlan Tsarnaev may have made other trips to the region. The congressman said Tsarnaev was "starting to radicalize" several years before the six-month trip.
"It's unconfirmed at this point. ... My judgment is we'll probably see there were other trips to the region," McCaul said.
Tsarnaev, 26, may have traveled under an alias when he went to Russia in 2012, House Intelligence Committee Chairman Rep. Mike Rogers said Sunday.
Rogers, R-Michigan, also told NBC that Tamerlan Tsarnaev may have become radicalized by Islamic extremists during that trip.
The Russian embassy tweeted Friday that the brothers were not on the consular registry at the embassy in Washington nor the general consulate in New York.
The brother: Hints of radicalization
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.
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 last three or four years, according to an analysis of his social media accounts and the accounts of family members. But so far, there is no evidence of active association with international jihadist groups.
In August 2012, soon after returning from his visit to Russia, the elder Tsarnaev 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.
But analysis by CNN and the SITE Intelligence Institute has uncovered a screen grab from one of those videos. It features members of the group Imarat Kavkaz, identifiable by the logo on their shirts. Imarat Kavkaz is the most potent militant Islamist group in the north Caucasus region, which includes Chechnya and Dagestan.
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.
Imarat Kavkaz has its roots in the 1990s Chechen insurgency. It was founded in 2007 to bring together various jihadist groups fighting to create an Islamic state in the region.
Its overall leader, Doku Umarov, is a veteran Chechen guerrilla who claimed responsibility for the 2011 bombing of Moscow's international airport.
Rebels who call themselves Mujahideen of the Caucasus Emirate Province of Dagestan issued a statement Sunday, 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.
So far, evidence suggests that the two brothers acted alone in the bombings and subsequent shootout, Watertown Police Chief Edward Deveau told CNN Saturday.
"From what I know right now, these two acted together and alone," the police chief 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."
What's next for the suspect?
Authorities have not publicly detailed how Dzhokhar Tsarnaev was injured, but a federal official said he has injuries to the throat.
An official who has been briefed on the case said he was "intubated and sedated."
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.
Secretary of Defense Chuck Hagel told reporters on a flight to Israel that the attack was "criminal," adding that "every region of the world is not safe from these terrible acts."
He also said he has not seen any intelligence that linked the brothers to any terrorist organization, but it was still early in the investigation.
Deveau, the Watertown police chief, said officers did not question the suspect immediately after he was found.
In another development, immigration agents arrested two people Saturday in New Bedford, Massachusetts, on visa issues, according to a statement from Immigration and Customs Enforcement. A law enforcement official said the two were detained as authorities investigated their possible involvement in the bombings case. It was determined they weren't involved, that official said, but they were arrested on administrative immigration violations, according to the ICE statement.
After the bombings, Tsarnaev went out to party
As an army of officers hunted for the suspects in Monday's marathon bombings, Dzhokhar Tsarnaev acted like any other college sophomore.
He was on the campus of University of Massachusetts Dartmouth every day after the attack until late Thursday, a university official told CNN. Tsarnaev attended classes and dorm parties while much of Boston was at a tense standstill.
A student at the school told The Boston Globe she saw Tsarnaev Wednesday night at a party that was attended by some of his friends from intramural soccer.
"He was just relaxed," she said, asking the paper not to print her name.
At the dorm where Tsarnaev lived, students joked Thursday as they viewed the FBI photos of the bombing suspects on television, a senior who lived in that dorm told The Boston Globe.
"We made a joke like, that could be Dzhokhar," Pamala Rolon said. "But then we thought it just couldn't be him. Dzhokhar? Never."
The campus, which was closed during the search for the bombing suspects, reopened Sunday morning.
Moment of silence
Meanwhile, Boston is trying to return to semblance of normalcy with some streets and business reopening.
Even so, federal investigators were still scouring the site of the bombings near the marathon's finish line on Boylston Street. The street remained closed Sunday.
People throughout Massachusetts are being urged to observe a moment of silence Monday at 2:50 p.m., exactly one week after the Boston Marathon bombings.
Boston Mayor Thomas Menino and Gov. Deval Patrick announced the planned moment on Sunday.
CNN's Tim Lister, Paul Cruickshank, Deborah Feyerick, Jill Dougherty, Pamela Brown, Julian Cummings, Barbara Starr, Ann O'Neill, Susan Candiotti, Jake Tapper, Shannon Travis, Lindy Royce and Drew Griffin contributed to this report.