(CNN) -- Boston Marathon bombing suspect Dzhokhar Tsarnaev made a "statement to his detriment" during a prison visit by his sister, federal prosecutors said in a court document.
The statement was made in the presence of an FBI agent, who was in the room during the visit under special restrictions, according to documents filed in federal court in Boston on Friday.
Tsarnaev, in the presence of the agent, "was unable to temper his remarks and made a statement to his detriment which was overheard by the agent." The document did not say what the statement was.
The suspect's alleged statement surfaced during the prosecution's opposition to a defense motion to do away with restrictions known as Special Administrative Measures, or "SAMs," which include that prison visits be monitored.
A defense motion to vacate the measures -- which was unsealed Friday -- describes the SAMs as "unlawful and unwarranted."
The measures were imposed in August and restrict Tsarnaev's access to mail, media, the telephone, and visitors.
The defense motion claims the SAMs "impose extraordinary and severe restrictions impairing the ability of defense counsel to provide competent representation."
Tsarnaev, 20, is being held without bail on federal charges in connection with the April 15 bombings that killed three people and wounded more than 250 others. He's also charged with the killing of an MIT police officer several days later.
His trial is set for November 3. Federal prosecutors said last month they'll seek the death penalty against Tsarnaev, arguing that he acted in "an especially heinous, cruel and depraved manner."
The government argued that the defense motion was not about the restrictions, but about the fact that Tsarnaev allegedly made a detrimental comment in front of an FBI agent, according to the court document.
The defense motion "has nothing to do with the SAMs and everything to do with the fact that 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 court document states.
The government said the FBI agent was in "a legally obtained vantage point making his ability to see and hear all which occurred lawful."
Authorities allege Tsarnaev, a Chechnya-born American, and his brother, Tamerlan, planted two homemade bombs near the finish line of the marathon before allegedly killing a police officer three days later.
The attacks triggered the massive manhunt. Police shot and killed the suspect's brother, Tamerlan Tsarnaev, during the manhunt.
Calls to Tsarnaev's attorneys were not immediately returned Saturday.