Tamerlan Tsarnaev, shown in a 2009 arrest photo, sought early last year to change his first name to Muaz, after slain Dagestan rebel fighter Emir Muaz. (Cambridge (Mass.) Police Department) 
 
 WASHINGTON — Less than three months before the Boston Marathon bombings, a bitter, frustrated Tamerlan Tsarnaev visited a federal immigration office in the Boston area and signed forms seeking to legally change his first name. 
 
 Eager to recast himself in the model of a well-known rebel figure killed by Russian forces in 2009, Tsarnaev chose as his new name "Muaz," according to a previously undisclosed Homeland Security Department petition form, which was obtained by The Times. It was not only a tribute to Emir Muaz, a celebrated fighter in Russia's Dagestan republic, it was also the nickname rebels had given Tsarnaev during his six-month visit to the region in 2012, law enforcement officials say. 
 
 Asked as part of the Jan. 23, 2013, application to explain his name change request, Tsarnaev described the decision in political terms, according to a federal law enforcement official close to the Boston bombing case. "He said, 'The Russian people have been terrorizing my home country for all these years.' This is why he needed to come back to America and help," the official said. 
 
 A year after twin explosive-laden backpacks killed three people and injured more than 260 others at the Boston Marathon, the name-change petition is part of a growing body of evidence that portrays Tsarnaev as more radical and organized than previously believed. 
 
 Defense attorneys for his younger brother, Dzhokhar Tsarnaev, say the older sibling may have been set off by what he believed was FBI pressure and attempts to recruit him as an informant. 
 
 The emerging picture of Tamerlan Tsarnaev, who was killed as U.S. agents tried to arrest him days after the bombings, is likely to play a key role in the upcoming defense of his brother. Attorneys for Dzhokhar Tsarnaev are expected to argue that his brother took the lead role in what became the worst domestic terrorist attack since Sept. 11, 2001. 
 
 U.S. authorities believe Tamerlan was deeply radicalized during his 2012 visit to Dagestan, which, along with adjoining Chechnya, is home to an Islamist insurgent movement. They say he made an unsuccessful attempt to join the rebels and was either sent back to the U.S. to carry out a terrorist strike or took it upon himself. 
 
 "You've got to be pretty full into this to want to change your name and not be just a nobody named Tamerlan," said the law enforcement source, who requested anonymity because the case is ongoing. "Maybe he thought because he could not get accepted over there, maybe he could do something here." 
 
 At the bottom of the name-change form, Tsarnaev was advised that he would be required to take an oath of allegiance to the United States, including potentially serving in the U.S. armed forces. Rather than sign that part, he printed his name, then crossed out "Tamerlan" and changed it to "Muaz." 
 
 Tsarnaev's anger toward the U.S. was fueled by what he perceived as pressure from American law enforcement agents to work for them, according to his brother's defense attorneys. 
 
 Though the FBI has said that it met once with Tsarnaev to discuss his visit to the Caucasus region, lawyers for his brother claimed in court filings March 28 that federal agents interviewed him multiple times, a process that they said increased his anger against the U.S. 
 
 In 2011, the Russian government asked the FBI to check on Tsarnaev, warning he might be preparing to travel to the Caucasus region. The FBI said it ran a series of checks, spoke with him and his parents, and reported back to Moscow that it had uncovered no links to terrorism. 
 
 But according to his brother's lawyers, the FBI's involvement went further. They said in court papers that there was "more than one" FBI visit to talk with Tsarnaev and his parents, that he was questioned about his Internet searches, and that the bureau "asked him to be an informant." 
 
 "Tamerlan misinterpreted the visits and discussions with the FBI as pressure, and they amounted to a stressor that increased his paranoia and distress," defense attorneys argued in the filings. 
 
 Federal prosecutors, however, told defense attorneys in a March 14 letter that they had "no evidence that Tamerlan Tsarnaev was solicited by the government to be an informant." They did not comment on whether there were multiple FBI visits. 
 
 The image of Tsarnaev as a more radicalized fighter fits with the conclusion of a House Committee on Homeland Security report last month, which said he returned to the U.S. in 2012 with a near-manic desire to strike out against the U.S., evidenced in tirades he carried out at local mosques. 
 
 Signs of his radicalization also included a YouTube account he posted under his name that included Russian-language videos on Islam and play lists of militant videos he compiled, the committee report said. 
 
 The new revelations about Tsarnaev have prompted defense lawyers for his brother to characterize him as the key player who "supplied the motivation, planning and ideology behind the Boston Marathon attack," according to recent filings. 
 
 Dzhokhar Tsarnaev has pleaded not guilty to using a weapon of mass destruction and other federal charges. If convicted at his trial this fall, he faces a death sentence. 
 
 His attorneys say Tamerlan "induced or coerced" Dzhokhar to take part in the attack and that the younger brother "acted under his domination and control." 
 
 J. Reid Meloy, a clinical professor of psychiatry at the UC San Diego School of Medicine, who has worked on notable terrorist cases including the Oklahoma City federal building bombing, said an "undue or coercive behavior" defense can be hard to prove and difficult to sell to a jury. 
 
 In an interview, Meloy said that one of the few examples in which it worked was the 2002 Washington-area sniper case in which 10 people were killed and three injured. An adult killer was put to death, while his teenage protege, who looked up to him as a mentor and father figure, was given life in prison without parole. 
 
 "In my view of the evidence in the Boston case, I find no indication that the younger man had a psychiatric disorder," Meloy said. But he added, "It now seems very clear that this may be the most viable and convenient defense for the younger Tsarnaev." 
 
 richard.serrano@latimes.com ||||| Lawyers for accused Boston Marathon bomber Dzhokhar Tsarnaev asserted Friday that his older brother and alleged accomplice had been encouraged by the FBI to be an informant and to report on the Chechen and Muslim community, according to court records. 
 
 “We seek this information based on our belief that these contacts were among the precipitating events for Tamerlan’s actions during the week of April 15, 2013, and thus material to the defense case in mitigation,” the lawyers said in their court filing. 
 
 “We base this on information from our client’s family and other sources that the FBI made more than one visit to talk with Anzor [his father], Zubeidat [his mother] and Tamerlan, questioned Tamerlan about his Internet searches, and asked him to be an informant, reporting on the Chechen and Muslim community. 
 
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 “We do not suggest that these contacts are to be blamed and have no evidence to suggest that they were improper, but rather view them as an important part of the story of Tamerlan’s decline. Since Tamerlan is dead, the government is the source of corroboration that these visits did in fact occur and of what was said during them.” 
 
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 The lawyers suggested that Tamerlan Tsarnaev could have misinterpreted his interactions with the FBI as pressure from the agency, and that they could have “increased his paranoia and distress.” 
 
 The defense wants to investigate those factors as it seeks to portray Tamerlan as a dominating family figure who may have pushed the younger Dzhokhar to take part in the April 15 bombings last year. Tamerlan was killed days after the bombings in a confrontation with police in Watertown. 
 
 The lawyers’ allegation, based on conversations with family members and other sources, was made in a 23-page court filing Friday in which the lawyers sought a court order forcing prosecutors to turn over more evidence in the case. 
 
 Specifically, they want additional evidence about Tamerlan’s radical views, which would be useful in the lawyers’ defense against the death penalty. 
 
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 Lawyers argued that, now that prosecutors have declared they will seek capital punishment, Dzhokar should be allowed to present evidence of mitigating factors that would tilt a jury against the death penalty. 
 
 Those factors include evidence about his age at the time of the bombings, which was 19, his lack of criminal history, and the possible influence of others, specifically a radical older brother, court records said. 
 
 “The underlying data concerning the brothers’ activities, state of mind, and respective trajectories is critical,” the defense said. “Evidence that shows Tamerlan to have had a substantially longer and deeper engagement than his younger brother with extremist and violent ideology is mitigating for the light that it sheds on their relative culpability.” 
 
 Prosecutors did not immediately respond to the defense request, though defense lawyers conceded that they have a March 14, 2014, letter in which prosecutors said they had “no evidence that Tamerlan Tsarnaev was solicited by the government to be an informant.” 
 
 Although the FBI does not comment on court matters, the bureau cited a statement Friday that it made Oct. 18, 2013, which said, “The Tsarnaev brothers were never sources for the FBI, nor did the FBI attempt to recruit them as sources.” 
 
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 Dzokhar Tsarnaev, now 20, faces a 30-count indictment in his alleged role in the bombings, which killed three people and injured more than 260. He and Tamerlan also allegedly shot and killed an MIT police officer before the confrontation with police in Watertown. 
 
 Tsarnaev is being held at the federal prison at Fort Devens in Ayer. He is slated to go to trial in November. 
 
 Prosecutors have described him as a young Muslim extremist who wanted to carry out jihad, or holy war, against the United States. They alleged the brothers learned to build the bombs through websites that supported Al Qaeda. 
 
 In court requests filed late Friday, the defense lawyers sought a court order to force prosecutors to turn over a list of records, including reports related to the fatal shooting of MIT police Officer Sean Collier, the Watertown confrontation, and any evidence that an MBTA police officer was shot by fellow officers, rather than by Dzokhar Tsarnaev, who the lawyers say was unarmed. 
 
 The request also targets records of the brothers’ Internet searches. 
 
 One of the court requests asked for any information prosecutors collected under the Foreign Intelligence Surveillance Act including any surveillance of Tamerlan’s return visits to Dagestan and the Chechnya region before the bombings. 
 
 A recent US House of Representatives homeland security report indicated FBI officials in Moscow may have collected “electronic communication” between Tamerlan and a jihadist named William Plotnikov. 
 
 “Any surveillance, evidence, or interviews showing that Tamerlan’s pursuit of jihad predated Dzhokhar’s would tend to support the theory that Tamerlan was the main instigator of the tragic events that followed,” the defense lawyers argued. 
 
 The lawyers are also seeking the immigration records of Tsarnaev’s family members, including his mother, father, and brother. 
 
 His father, Anzor, sought the family’s asylum in fleeing torture in Kyrgyzstan close to two decades ago, and the defense lawyers argued that the records would illustrate Tsarnaev’s family history. 
 
 They also said that Tamerlan’s immigration records could yield more evidence about his encounters with federal authorities, specifically after his visits to Dagestan that caught the attention of Russian officials. 
 
 The lawyers said they plan to argue that the unknowns about Tsarnaev’s “formative environment and relative moral culpability” and the possible “psychological domination” by his brother would support their case against the death penalty. 
 
 Globe correspondent Haven Orecchio-Egresitz contributed to this report. Milton J. Valencia can be reached at MValencia@globe.com.
– On Jan. 23, 2013, less than three months before the Boston Marathon bombings, Tamerlan Tsarnaev was ready to reinvent himself. He applied that day to change his first name to "Muaz," in honor of a rebel from Russia's Dagestan republic who was killed by Russian forces in 2009. Muaz was also the nickname bestowed upon Tsarnaev when he visited Dagestan in 2012, officials say. The previously unreleased name change application was recently obtained from the Department of Homeland Security by the Los Angeles Times. The reason Tsarnaev gave for changing his name, according to an official who spoke to the Times: "He said, 'The Russian people have been terrorizing my home country for all these years.' This is why he needed to come back to America and help." This detail, and others that are emerging, paint the elder Tsarnaev as more radical than he was originally believed to be—and defense attorneys for younger brother Dzhokhar are likely to use that picture in court, to argue that Tamerlan was the leader and coerced his brother to participate. Dzhokhar's lawyers have argued that Tamerlan may have become increasingly "distressed" after the FBI interviewed him about the trip to Dagestan, possibly believing the bureau was pressuring him "to be an informant, reporting on the Chechen and Muslim community," the Boston Globe reported last month.