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WASHINGTON ­— The surviving suspect in the Boston Marathon bombings told FBI interrogators that he and his brother had considered suicide attacks and striking on the Fourth of July as they plotted their deadly assault, according to two law enforcement officials.

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 al-Awlaki before his death.

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 U.S. 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 about what 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 might have played in the plot or 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 Tsarnaev’s wife, Katherine Russell. According to two other law enforcement officials who were also granted anonymity because they did not want to be identified discussing an ongoing investigation, Russell’s fingerprints and DNA do not match those found on the fragments.

Federal authorities are skeptical of Russell’s insistence that she played no role in the attack or did not help the brothers elude the authorities after the FBI released photos of them. That skepticism has been stoked by Russell’s decision in recent days to stop cooperating with the authorities.

The FBI has also decided to send more agents to Russia to assist with the investigation, officials said. The bureau has been relying on a couple of agents it has based in the U.S. Embassy in Russia to serve as an intermediary with the authorities there.

U.S. and Russian investigators in Dagestan, in the turbulent Caucasus region of southern Russia, have been trying to determine what Tamerlan Tsarnaev did during a six-month visit to Dagestan last year. On Thursday, Rep. William Keating, D-Mass., said the investigators believed that Tsarnaev met with one known militant, Mahmoud Mansur Nidal, as first reported in the Russian newspaper Novaya Gazeta.

“I’m comfortable that on that trip he reached out to members of the insurgency in Dagestan,” said Keating, a member of the House Homeland Security Committee.

He said there was no evidence so far that Tamerlan 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.

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 that 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.

Separately, Dzhokhar Tsarnaev has told investigators that he and his brother had learned to build the pressure-cooker bombs from reading Inspire, the online magazine published by al-Qaeda 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.

The new details of what Dzhokhar Tsarnaev has told authorities fill out a growing portrait of what the grievously wounded young man told investigators from his hospital bedside.

In the course of questioning him about whether he knew of any other active plots or threats to public safety, Tsarnaev also admitted that he had been involved in laying the bombs that killed three people and injured more than 260 at the marathon. He told investigators that he knew of no other plots and that he and his brother had acted alone. He said he knew of no more bombs that had not been detonated.

Since then, investigators have been seeking to verify Tsarnaev’s statements as part of the investigation 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.

William K. Rashbaum and Serge Schmemann contributed reporting from New York.