But the suspect, Dzhokhar Tsarnaev, 19, told investigators that he and his brother, Tamerlan, 26, who was killed in a shootout with the police, had ultimately decided to use pressure-cooker bombs and other homemade explosive devices, the officials said.
The brothers finished building the bombs in Tamerlan's apartment in Cambridge, Mass., more quickly than they had anticipated and so decided to accelerate their attack to the Boston Marathon on April 15, Patriots' Day in Massachusetts, from July, according to the account that Dzhokhar provided authorities. They picked the finish line of the marathon after driving around the Boston area looking for alternative sites, according to this account.
In addition, Dzhokhar Tsarnaev has told authorities that he and his brother viewed the Internet sermons of Anwar al-Awlaki, a radical American cleric who moved to Yemen and was killed in September 2011 by a U.S. drone strike. There is no indication that the brothers had communicated with Awlaki.
Tsarnaev made his admission April 21, two days after he was captured while hiding in a boat in a nearby backyard, to specially trained FBI agents who had been waiting outside his hospital room for him to regain consciousness.
After he woke up, they questioned him, invoking what is known as the public safety exception to the Miranda Rule, a procedure authorized by a 1984 Supreme Court decision that in certain circumstances allows interrogation after an arrest without notifying a prisoner of the right to remain silent.
The new details of what Dzhokhar Tsarnaev has told the authorities emerged as the FBI moved forward Thursday with trying to determine how the brothers were radicalized and the role that Tamerlan Tsarnaev's wife, Katherine Russell, might have played in the plot or in helping the brothers evade the authorities after the attacks.
As part of those efforts, the authorities have sought to determine whether fingerprints and DNA found on bomb fragments were from Russell. According to two other law enforcement officials, Russell's fingerprints and DNA do not match those found on the fragments. All of the law enforcement officials were granted anonymity because they did not want to be identified discussing a continuing investigation.
Federal authorities are skeptical of Russell's insistence that she played no role in the attack or in helping the brothers elude the authorities after the FBI released photographs of them. That skepticism has been stoked by Russell's decision in recent days to stop cooperating with the authorities.
A funeral home retained by family members of Tamerlan Tsarnaev claimed his body from the Massachusetts state medical examiner's office about 5:30 p.m. Thursday, a spokesman for the Boston Department of Public Safety, Terrel Harris, said. He offered no other details.
The details of what Dzhokhar Tsarnaev told authorities fill out a growing portrait of what the grievously wounded young man has told investigators at his hospital bedside. During the interrogation, Tsarnaev, who sustained a gunshot wound to the neck, had trouble speaking and answered several questions by writing on a piece of paper and nodding.
In the course of the questioning, he acknowledged having laid the bombs that killed three people and injured more than 260 at the finish line of the marathon. He told the interrogators that he and his brother had been motivated to strike against the United States partly because of its military actions in Iraq and Afghanistan, and what the two saw as a broader conspiracy against Muslims. Tsarnaev also said that he knew of no other plots and that he and his brother had acted alone and were not connected to a larger terrorist network, according to the account.
While the authorities have found Tsarnaev's statements in his interrogation to be helpful, they are not treating them as fact and are seeking to confirm the details from other sources. The authorities have looked at everything from items the brothers 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.
As the investigation has moved forward in the United States, the FBI has been questioning witnesses in Russia and meeting with Russian law enforcement and intelligence officials. In recent days, the FBI decided to send more agents to Russia to assist with the investigation, officials said. The bureau has been relying on a small number of agents based in the U.S. Embassy in Moscow to serve as intermediaries with the authorities there.
U.S. and Russian investigators are particularly interested in what Tamerlan Tsarnaev did during a six-month visit last year to Dagestan, the turbulent Caucasus region of southern Russia. On Thursday, Rep. William Keating, D-Mass., said investigators believed that Tsarnaev had met with one known militant, Mahmoud Mansur Nidal, while he was there. The statements from Keating confirmed a report in the Russian newspaper Novaya Gazeta.
"I'm comfortable that on that trip he reached out to members of the insurgency in Dagestan," Keating, a member of the House Committee on Homeland Security, said of Tamerlan Tsarnaev. He said there was no evidence so far that Tsarnaev had succeeded in formally joining the insurgency, led by a group calling itself the Caucasus Emirate, or that he had received explosives training in Dagestan.
Russian government authorities contacted the FBI and Central Intelligence Agency in 2011, saying that they had information that Tamerlan Tsarnaev and his mother had ties to extremists. According to a statement released last month by the FBI, the Russians said that Tamerlan Tsarnaev "was a follower of radical Islam and a strong believer." The Russians said they had information that he had "changed drastically since 2010 as he prepared to leave the United States for travel to the country's region to join unspecified underground groups."
In response to the Russian requests, the FBI conducted a review of Tsarnaev, including checking a website he had visited and searching databases on terrorists. The FBI questioned him and members of his family. Ultimately the FBI told the Russians that it had no evidence that he had ties to extremists. The CIA came to a similar conclusion.
The Russians did not follow up on subsequent requests from the FBI for more information on Tsarnaev and his mother.
At a news conference Tuesday, President Barack Obama did not rule out a foreign link but suggested that the Tsarnaev brothers appeared to be "self-radicalized" and that local, homegrown terrorist plots were harder to detect and prevent than those originating overseas.
Obama said U.S. counterterrorism efforts had put pressure "on these networks that are well-financed and more sophisticated and can engage and project transnational threats against the United States," but that "one of the dangers that we now face are self-radicalized individuals who are already here in the United States."
Investigators believe that the views of the two brothers grew more radical over time and were influenced at least partly by the Internet sermons of Awlaki. In the interrogation of Tsarnaev, he told the authorities that he and his brother had learned to build the pressure-cooker bombs from reading Inspire, the online magazine published by al-Qaida in the Arabian Peninsula.
The magazine's first issue - which included an article titled "Make a Bomb in the Kitchen of Your Mom" - gave instructions about how to carry out crude, low-cost terrorist attacks.
The man officials have identified as the creative force behind Inspire, a U.S. citizen named Samir Khan, was killed in the same missile strike in Yemen that killed Awlaki.