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Lawyers for Dzhokhar Tsarnaev, accused of planting one of the bombs at last year's Boston Marathon, said Friday that repeated contacts by the FBI were among "precipitating events" leading up to the bombings.

In court documents filed Friday, defense lawyers asked the judge overseeing the case for an order requiring the government to turn over all records of the FBI's contacts with Tsarnaev's older brother, Tamerlan. He was interviewed by FBI agents in response to a concern expressed by the Russian government that he might become radicalized.

"We seek this information based on our belief that these contacts were among the precipitating events for Tamerlan's actions during the week" of the bombings, the lawyers aid.

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They wrote that the Tsarnaev family and "other sources" have said that the FBI made more than one visit to talk to Tamerlan Tsarnaev and his parents, during which agents "questioned Tamerlan about his internet searches, and asked him to be an informant, reporting on the Chechen and Muslim community."

The court filing quotes prosecutors as saying they have no evidence that the older Tsarnaev was asked to be an informant.

Nonetheless, the defense lawyers say they believe that Tamerlan misinterpreted the visits by the FBI as pressure, increasing his paranoia and distress.

"We do not suggest that these contracts are to be planed and have no evidence to suggest that they were improper," the lawyers say.

The FBI's interview took place in 2011.

The court document makes clear that his lawyers will stress the age difference between the younger Tsarnaev and his older brother, who were 19 and 26 at the time of the bombing. The lawyers say they will resist any attempt to persuade the jury "that the brothers were equally culpable, despite the marked differences in their ages, personalities, and levels of prior involvement in violent activity."

No trial date has yet been set.

—Pete Williams