WASHINGTON — 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 are still in the process of corroborating some of the admissions that officials have said were made by the surviving suspect in the attacks, Dzhokhar Tsarnaev, 19.
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 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. 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, the New York Times reported, citing unnamed law enforcement officials.
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 in a gun battle with police.
The brothers may have been planning the attacks for several months. On Feb. 6, Tamerlan Tsarnaev bought fireworks at a Phantom Fireworks store in Seabrook, N.H., 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.
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.