'Detrimental' statement by Boston Marathon bomb suspect claimed

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".