Dzhokhar Tsarnaev, the surviving suspect in the Boston Marathon bombings, has told investigators that his brother's widow has not been involved in any way in the terror plot.

Recently, FBI searched the computer of Katherine Russell, 24, and insider sources have reported that materials related to radical Islam have been found on it.

The FBI has removed evidence, including DNA samples, from the Rhode Island home of Russell's parents, where she has been staying since her husband's death.

Her lawyers say she did not know much about her husband's activities because she spent most of her time outside the home working as a health aide while he watched their child.

It is also reported that Russell, who converted to Islam under Tamerlan's influence, is not cooperating with the authorities.

Tamerlan Tsarnaev, 26, died after a gun battle with police during which he was run over by his younger brother as he fled the scene in a vehicle.

Tamerlan lived with his wife and their young daughter in an apartment in Cambridge, Massachusetts. Police have said they found bomb material at the residence.

The surviving brother, Dzohar, faces a possible death sentence if convicted for his alleged role in the Boston attack.

Three of Dzhokhar Tsarnaev's college friends appeared in court last week, accused of obstructing the police investigation into the attacks.

Police say Azamat Tazhayakov and Dias Kadyrbayev - both from Kazakhstan - threw away the suspect's backpack after realizing he was one of the bombers.

Robel Phillipos, a US citizen, is accused of lying to investigators.

The Tsarnaev family has origins in the predominantly Muslim republic of Chechnya in southern Russia.