BOSTON (AP) — An FBI agent overheard Boston Marathon bombing suspect Dzhokhar Tsarnaev make a "statement to his detriment" when his sister visited him in prison, federal prosecutors said Friday.

Prosecutors did not reveal what Tsarnaev said, but they objected to what they called an attempt by Tsarnaev's lawyers to suppress the statement.

Tsarnaev made the remark when an investigator working for his lawyers accompanied Tsarnaev's sister to a prison visit, a meeting that was monitored by an FBI agent, prosecutors said. The defense investigator started to explain to Tsarnaev's sister the rationale behind special restrictions placed on Tsarnaev in prison, prosecutors said.

They say Tsarnaev, "despite the presence of an FBI agent and an employee of the federal public defender, was unable to temper his remarks and made a statement to his detriment which was overheard by the agent."

The government described the conversation in a memo outlining their opposition to a request from Tsarnaev's lawyers to lift the prison restrictions, known as special administrative measures.

Tsarnaev, 20, has pleaded not guilty in the terrorist attack at last year's marathon. Two pressure cooker bombs were placed near the marathon finish line, killing three people and wounding more than 260.

Prosecutors have said they will seek the death penalty against Tsarnaev for crimes that include using a weapon of mass destruction.

In a separate filing late Friday, lawyers for Tsarnaev sought to have multiple charges against him dismissed, saying they are repetitive and that the total number of charges could sway jurors weighing whether to find him guilty and, if they do, whether to sentence him to death.

Noting that more than half of the 30 federal charges carry a possible death sentence, his lawyers wrote that the number of capital charges "appears designed to put a thumb on the scales of justice in favor of the death penalty."

Experts have said earlier filings suggest the defense may try to save Tsarnaev's life by arguing he fell under the influence of his brother. Friday's motion was submitted by his lawyer Judy Clarke, one of the nation's foremost death penalty specialists.

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Tsarnaev's lawyers have also argued that the prohibitive prison measures limit Tsarnaev's interactions with people helping his defense team.

Prosecutors argued that the FBI agent's presence was permitted by the special administrative measures, which prohibit providing information to people outside the prison.

Judge George O'Toole Jr. agreed to ease some of the restrictions earlier, but Tsarnaev's lawyers filed a new request last week to lift them.

Miriam Conrad, one of Tsarnaev's lawyers, said their renewed request "was not prompted by any comment that Mr. Tsarnaev made during a family visit."

"We have continuously opposed the SAMs (special administrative measures) as unwarranted, and we believe they unduly interfere with our representation of our client."

Tsarnaev's lawyers had complained that the restrictions "continue to interfere with preparation of the defense in important ways," including "obstacles related to FBI monitoring of family visits."

Tsarnaev's lawyers say the presence of the FBI agent during prison visits by Tsarnaev's two sisters "has thwarted the defense ability to develop important mitigation information."

They argue that courts have recognized the kind of information the defense wants to develop as admissible mitigation evidence, including evidence on "family dysfunction, mental illness and the impact of family chaos on the defendant as he grew up."

Prosecutors have argued that the restrictions are necessary in Tsarnaev's case because of his "commitment to jihad" and his "widespread notoriety."

"There was no expectation of privacy on the part of Tsarnaev, his visitors or the investigator," they argue.

Prosecutors have alleged that Tsarnaev and his brother, Tamerlan Tsarnaev, 26, built the bombs and placed them near the finish line of the April 15 marathon. Tamerlan Tsarnaev died following a shootout with police several days after the bombing.

A spokeswoman for U.S. Attorney Carmen Ortiz declined to comment.