Boston bomber says big brother was the mastermind

Facing the death penalty on charges of using a weapon of mass destruction, the surviving Boston bombings suspect has claimed that his older brother, not any international terrorist group, masterminded the deadly attack, according to a media report.

Dzhokhar Tsarnaev, 19, who has been hospitalised with a tube down his throat in recent days, communicated with investigators by writing and nodding, CNN reported citing a US government source.

The preliminary interviews with Tsarnaev, who had escaped a shootout with police that killed the second suspect, his older brother Tamerlan, 26, early Friday morning, suggest the two brothers fit the classification of self-radicalised jihadists, the unnamed source was quoted as saying.

Tsarnaev has conveyed to investigators that Tamerlan's motivation stemmed from jihadist thought and the idea that Islam is under attack, and jihadists need to fight back, the source told CNN.

But he cautioned that the interviews were preliminary, and that Tsarnaev's account needs to be checked out and followed up on by investigators.

CNN also cited a federal law enforcement official as saying that while investigators have seen no indication the suspects collaborated with others, they still have plenty of work to do before they can confidently say no others were involved.

Investigators have been asking Tsarnaev whether there are more bombs, explosives caches or weapons beyond those already found by police, and if anyone else was involved in the attacks, a source with direct knowledge of the investigation told CNN.

Investigators are going into Tsarnaev's room every few hours to ask questions in the presence of doctors, the channel said.

Initial proceedings against the younger Tsarnaev were conducted Monday at Boston's Beth Israel Deaconess Medical Centre by a federal magistrate with two assistant US attorneys, including Indian American Aloke Chakravarty, appearing for the prosecution.

Chakravarty and William Weinreb from the anti-terrorism and national security unit of the US Attorney's office in Boston, will prosecute Tsarnaev, who remains under armed guard in serious condition, the Justice Department announced Monday.