In Washington, Senate Intelligence Committee member Richard Burr, R-N.C., said after his panel was briefed by federal law enforcement officials that there is "no question" that Tamerlan Tsarnaev, 26, was "the dominant force" behind the attacks, and that the brothers had apparently been radicalized by material on the Internet rather than by contact with militant groups overseas.
Martin Richard, a schoolboy from Boston's Dorchester neighborhood who was the youngest of those killed in the April 15 blasts at the marathon finish line, was laid to rest after a family-only funeral Mass.
"The outpouring of love and support over the last week has been tremendous," the family said in a statement. "This has been the most difficult week of our lives."
A funeral was also held for Massachusetts Institute of Technology police officer Sean Collier, 26, who authorities said was shot to death by the Tsarnaev brothers three days after the bombing. A memorial service for Collier was scheduled for Wednesday at MIT, with Vice President Joe Biden expected to attend.
More than 260 people were injured by the bomb blasts. About 50 were still hospitalized.
Authorities believe neither brother had links to terror groups. However, two U.S. officials said Tuesday that Tamerlan Tsarnaev who died last week in a gunbattle frequently looked at extremist websites, including Inspire magazine, an English-language online publication produced by al-Qaida's Yemen affiliate. The magazine has endorsed lone-wolf terror attacks.
Both officials spoke on condition of anonymity because they were not authorized to discuss the investigation publicly.
On Capitol Hill, members of the Senate Intelligence Committee were briefed by the FBI and other law enforcement officials at a closed-door session Tuesday evening.
Afterward, Sen. Marco Rubio, R-Fla., described the two brothers as "a couple of individuals who become radicalized using Internet sources."
"So we need to be prepared for Boston-type attacks, not just 9/11-style attacks," Rubio said, referring to lone-wolf terrorists as opposed to well-organized teams from established terror networks.
Sen. Saxby Chambliss, R-Ga., said law enforcement officials have gotten "minimal" information from Dzhokhar Tsarnaev and are still looking into whether the brothers had training or coaching from a foreign group.
The brothers' parents live in Dagestan, a predominantly Muslim province in Russia's Caucasus, where Islamic militants have waged an insurgency against Russian security forces for years.
Family members reached in the U.S. and abroad by The Associated Press said Tamerlan was steered toward a strict strain of Islam under the influence of a Muslim convert known to the Tsarnaev family only as Misha.
After befriending Misha, Tamerlan gave up boxing, stopped studying music and began opposing the wars in Afghanistan and Iraq, according to family members, who said he turned to websites and literature claiming that the CIA was behind 9/11.
"Somehow, he just took his brain," said Tamerlan's uncle, Ruslan Tsarni of Montgomery Village, Md., who recalled conversations with Tamerlan's worried father about Misha's influence.
"You could always hear his younger brother and sisters say, 'Tamerlan said this,' and 'Tamerlan said that.' Dzhokhar loved him. He would do whatever Tamerlan would say," recalled Elmirza Khozhugov, the ex-husband of Tamerlan's sister. He spoke by telephone from his home in Almaty, Kazakhstan.
The brothers, who came to the U.S. from Russia a decade ago, were raised in a home that followed Sunni Islam, the religion's largest sect, but were not regulars at the mosque and rarely discussed religion, Khozhugov said.
Then, in 2008 or 2009, Tamerlan met Misha, a heavyset bald man with a reddish beard. Khozhugov didn't know where they met but believed they attended a Boston-area mosque together.
Hoping to learn more about the Tsarnaev brothers' motives, U.S. investigators traveled to southern Russia on Tuesday to speak to their parents, a U.S. Embassy official said.
A lawyer for the family, Zaurbek Sadakhanov, said the parents had just seen pictures of the mutilated body of their elder son and were not up to speaking with anyone.
In Massachusetts, the state House turned aside a bid by several lawmakers to reinstate the death penalty in certain cases, including the murder of police officers. In a 119-38 vote, the House sent the proposal to a study committee rather than advance it to an up-or-down vote.
In another development, April Walton, the manager of Phantom Fireworks of Seabrook, N.H., said Tamerlan Tsarnaev bought 48 mortar shells at the store in February.
Company Vice President William Weimer said FBI agents visited the store on Friday, interviewed staff and checked its computers. He said the amount of gunpowder that could be extracted from the fireworks would not have been enough for the Boston bombs.
Lawyers: Bombing suspect's wife assisting inquiry
Lawyers for the wife of the deceased Boston Marathon bombing suspect say she is doing everything she can to assist authorities.
But they wouldn't say Tuesday if Katherine Tsarnaeva, widow of Tamerlan Tsarnaev, has spoken to investigators yet.
Providence attorneys Amato DeLuca and Miriam Weizenbaum issued a statement Tuesday saying Tsarnaeva is deeply mourning the bombing victims. They say that Tsarnaeva and her family were in shock when they learned of allegations against her husband and brother-in-law, Dzhokhar Tsarnaev.
The lawyers say Tsarnaeva, whose toddler is the daughter of the late suspect, is "trying to come to terms with these events."
DeLuca told the Associated Press on Sunday that federal investigators want to speak with his client. They wouldn't comment Tuesday beyond the statement.