An FBI agent overheard Boston Marathon bombing suspect Dzhokhar Tsarnaev make a "statement to his detriment" when his sister visited him in prison, US prosecutors said.
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, 20, has pleaded not guilty in the attack at last year's marathon. Two pressure-cooker bombs were placed near the 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.
The defendant 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 defence investigator started to explain to the 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 its opposition to a request from Tsarnaev's lawyers to lift the prison restrictions, known as special administrative measures.
In a separate filing on 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 whether to sentence him to death.
They noted that more than half of the 30 federal charges carry a possible death sentence.