THE suspect in the Boston Marathon bombing admitted to a role in the attack during an interrogation in hospital a day after his arrest, US prosecutors have said.

That interview and other statements in the hours after Dzhokhar Tsarnaev's arrest should be admitted at his upcoming trial because the agents interviewing him believed there was a risk of further attacks, prosecutors said in filings to the Boston District Court.

While the suspect, now 20, insisted that he and his older brother Tamerlan - who died after a gunfight with police days after the April 15, 2013 bombing that killed three people and injured 264 - were the only people involved, investigators did not believe him, prosecutors said.

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"Agents continued to question Tsarnaev not to extract a confession, which they already had, but because of their reasonable belief that he was concealing information about impending attacks and/or additional bombs," prosecutors said.

Tsarnaev's lawyers argued the statements should not be admitted as he did not have an attorney present.