The portrait investigators have begun to piece together of the two brothers suspected of the Boston Marathon bombings suggests that they were motivated by extremist Islamic beliefs but were not acting with known terrorist groups -- and that they may have learned to build bombs simply by logging onto the online English-language magazine of the al-Qaida affiliate in Yemen, law enforcement officials said Tuesday.
The investigation into the bombings is still in its earliest stages, and federal authorities were still in the process of corroborating some of the admissions that law enforcement officials said were made by the surviving suspect in the attacks, Dzhokhar Tsarnaev, 19. But they said some of his statements suggested that the two brothers could represent the kind of emerging threat that federal authorities have long feared: angry and alienated young men, apparently self-trained and unaffiliated with any particular terrorist group, able to use the Internet to learn their lethal craft.
Sen. Marco Rubio, R-Fla., who serves on the Senate Intelligence Committee, told reporters after emerging from a two-hour classified briefing with FBI and intelligence officials Tuesday evening that the suspects were most likely radicalized over the Internet, but that investigators were still searching for possible sources of inspiration or support overseas.
"The increasing signals are that these were individuals who were radicalized, especially the older brother, over a period of time -- radicalized by Islamist fundamentalist terrorists, basically using Internet sources to gain not just the types of philosophical beliefs that radicalized them, but also learning components of how to do these sorts of things," Rubio told reporters.
"This is a new element of terrorism that we have to face in our country," Rubio said. "We need to be prepared for Boston-type attacks, not just 9/11-type attacks."
Dzhokhar Tsarnaev admitted to playing a role in the marathon bombings, which killed three people and wounded more than 260, and told federal agents that he and his brother were motivated by extremist Islamic beliefs, when he was interviewed Sunday at the hospital, law enforcement officials said.
Tsarnaev, who was recovering from gunshot wounds he received Friday while trying to elude the police, said that he and his brother had not been acting with any terrorist groups, the officials said, and told the investigators that they had learned about building explosive devices from Inspire, the online English magazine of the al-Qaida affiliate in Yemen.
Now investigators will try to check Tsarnaev's statements as they conduct a wide-ranging inquiry into the lives of the two brothers, speaking with people who knew them and looking at everything from items they left behind in their homes and, in the case of Dzhokhar Tsarnaev, his dorm room, to the lengthy digital trail they left through their emails and posts on social media sites. Investigators are still interested in a trip that his older brother, Tamerlan Tsarnaev, 26, made to Dagestan and Chechnya last year.
Tamerlan Tsarnaev was killed early Friday morning after he was shot by the police and struck by his brother's car as his brother escaped, law enforcement officials said.
One law enforcement official said that investigators were interested in learning whether Tamerlan Tsarnaev's wife, Katherine Russell, knew anything about the bombings.
"At one point, we were looking very hard at her, but less so now," the official said. "But we are still looking at her."
A lawyer for Russell released a statement on Tuesday saying that reports about her husband and her brother-in-law's involvement in the Boston Marathon bombing came as "an absolute shock." The lawyer, Amato A. DeLuca, also said that Russell, who grew up in North Kingstown, R.I., and met Tsarnaev when she was a college student at Suffolk University, was "doing everything she can to assist with the investigation."
Martin Richard, the 8-year-old boy killed in the bombings, was remembered at a private funeral with his immediate family on Tuesday morning. "This has been the most difficult week of our lives, and we appreciate that our friends and family have given us space to grieve and heal," his parents, Bill and Denise, said in a statement.
And in Stoneham, Mass., roughly 12 miles outside Cambridge, hundreds of mourners attended a funeral Mass for Sean A. Collier, a campus police officer at the Massachusetts Institute of Technology who was shot and killed last Thursday night.
A law enforcement official said that the brothers had been linked to Collier's shooting by a man they carjacked later Thursday night. The brothers told the carjacking victim that they had killed a police officer and committed the marathon bombings, the official said.
Investigators believe that the officer was killed in an attempt to get his gun, the official said, adding that there were indications that the men had tried -- but failed -- to remove it from its triple-lock holster.
When Dzhokhar Tsarnaev spoke to investigators on Sunday, officials said, he indicated that he and his brother had learned to make the pressure-cooker bombs that they used at the marathon from Inspire, the online al-Qaida magazine.
The magazine's first issue came out in mid-2010, and contained bomb-making instructions in articles with titles like "Make a Bomb in the Kitchen of Your Mom." Intelligence officials identified Samir Khan, a 24-year-old American, as the editor of the publication.
Khan, who grew up in Queens and North Carolina, proclaimed in the magazine in 2010 that he was "proud to be a traitor to America." He was killed in the drone strike in Yemen in September 2011 that killed Anwar al-Awlaki, the radical Yemeni-American cleric who officials said was one of the al-Qaida group's top leaders and propagandists.
The brothers may have been planning the marathon attacks for several months. On Feb. 6, Tamerlan Tsarnaev bought fireworks at a Phantom Fireworks store in Seabrook, N.H., about an hour's drive north of Boston, said William Weimer, the vice president of Phantom Fireworks, which is based in Youngstown, Ohio, and has 68 stores in 15 states.
"He came in and he asked the question that 90 percent of males ask when they walk into a fireworks store: 'What's the most powerful thing you've got?'" Weimer said in a telephone interview, adding that the store's clerk had described the sale as "uneventful."
Tsarnaev settled on a reloadable mortar kit called a Lock and Load, which comes with a launch tube and shells, Weimer said. But Weimer said that even if the brothers had harvested all the powder from the shells Tsarnaev bought that day, he did not believe it would have yielded enough explosives to make the two pressure cooker bombs that exploded on Boylston Street and the other devices that the suspects had with them when they were chased by the police early Friday morning.
Weimer said that his company, which sold fireworks in 2010 to Faisal Shahzad, who unsuccessfully tried to detonate a car bomb in Times Square, had checked its records for Tsarnaev's name as soon as it was made public, and had given the information to the FBI.
Elmirza Khozhugov, 26, the ex-husband of Tamerlan Tsarnaev's younger sister, Ailina, said that Tamerlan Tsarnaev had been enamored of conspiracy theories, and that he was also concerned by the wars in the Middle East.
"He was looking for connections between the wars in the Middle East and oppression of Muslim population around the globe," Khozhugov said in an email. "It was very hard to argue with him on themes somehow connected to religion. On the other hand he did not hate Christians. He respected their faith. Never said anything bad about other religions. But he was angry that the world pictures Islam as a violent religion."
Dzhokhar Tsarnaev's court-appointed lawyers took their first formal steps Tuesday toward preparing his defense, filing a motion to request that the court provide them with additional expertise in death penalty cases.
Miriam Conrad, of the Federal Public Defender's Office, asked U.S. Magistrate Judge Marianne B. Bowler to appoint two lawyers "learned in the law applicable to capital cases."
Outside the home of the Tsarnaev brothers' parents in Makhachkala, Russia, friends of the family told reporters Tuesday afternoon that their mother, Zubeidat Tsarnaeva, had grown distraught after seeing a photograph of Tamerlan Tsarnaev, on television.
Kheda Saratova, a well-known Chechen human rights activist and friend of the family, told reporters: "Please don't torture this family, they want to wait awhile, they are in terrible grief. Please."
"We must defend this family while the case is being investigated, so we can't say anything for now," she said.
Zubeidat Tsarnaeva, who has given a number of interviews in recent days, walked out from behind her wearing a bright yellow head scarf, and made her way, through a scrum of photographers and reporters, to hail a taxi. "My son is just my son," she said in English.