"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.
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.
A CNN analysis of the YouTube channel has established that 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 Makhachkala, the capital of 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.
'He was just relaxed'
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 went to the gym and dorm parties while much of Boston was at a tense standstill.
On Tuesday, he and a fellow student were at the campus gym when the bombings came up in discussion.
Zach Bettencourt said he expressed his shock about the attack, telling Tsarnaev that it was the type of thing you hear about in Iraq or Afghanistan, not Boston.
He said Tsarnaev responded: "Yeah, tragedies happen man. Like these things happen around the world. It's crazy."
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.