New details emerged Tuesday from U.S. officials and family members about how the two Boston Marathon bombing suspects may have been swayed by a radical, anti-American strain of Islam. A U.S. senator said they had been radicalized by sources on the web, not through direct contact with terror groups.

The condition of the younger brother, Dzhokhar Tsarnaev, was upgraded from serious to fair as investigators continued building their case against the 19-year-old university student. He could face the death penalty after being charged Monday with joining forces with his brother, now dead, in setting off the shrapnel-packed pressure-cooker bombs that killed three people.

More than 260 people were injured by the bomb blasts last week. About 50 were still hospitalized.

In Washington, Republican Sen. Richard Burr said after the Senate Intelligence Committee was briefed by federal law enforcement officials that there is "no question" that older brother Tamerlan Tsarnaev 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.

Authorities believe neither brother, both Russian-born ethnic Chechens, had links to terror groups. However, two U.S. officials said Tuesday that Tamerlan Tsarnaev, 26 — 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.

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 the Sept. 11, 2001, attacks.

"Somehow, he just took his brain," said Tamerlan's uncle, Ruslan Tsarni of Maryland, 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.

U.S. investigators travel to Russia

Hoping to learn more about the motives, U.S. investigators traveled to southern Russia on Tuesday to speak to the parents of the two suspects, a U.S. Embassy official said.

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Katherine Tsarnaeva, widow of Boston Marathon bombing suspect Tamerlan Tsarnaev, leaves the law office of DeLuca and Weizenbaum on Tuesday in Providence, R.I. Her lawyers say she is assisting authorities in the case. (Stew Milne/Associated Press)

The 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.

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 Washington, members of the Senate intelligence committee were briefed by the FBI and other law enforcement officials at a closed-door session Tuesday evening.

Afterward, Republican Senator Marco Rubio 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.

Republican Senator Saxby Chambliss 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.

Funerals held for Martin Richard, Sean Collier

The Boston area continued mourning the victims of the attack Tuesday. A funeral was held for Martin Richard, an eight-year-old schoolboy from Boston's Dorchester neighbourhood who was the youngest of those killed in the April 15 blasts at the marathon finish line.

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Martin Richard, 8, was the youngest of three people killed last week in the bombings at the Boston Marathon. His family had a private funeral Tuesday. (Bill Richard/Associated Press)

"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 on April 18. A memorial service for Collier was scheduled for Wednesday at MIT, with Vice-President Joe Biden expected to attend.

Also Tuesday, the Massachusetts House of Representatives 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.