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.
“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.”
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.
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.”
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.