Bags of evidence were taken from the home in Rhode Island where the widow of suspected Boston Marathon bomber Tamerlan Tsarnaev has been living.

FBI agents spent hours at the home of Katherine Russell's parents in North Kingstown and came out carrying bags marked 'DNA samples', a person familiar with the case said.

Investigators are hunting for evidence that Tamerlan Tsarnaev and his younger brother Dzhokhar Tsarnaev made and set off two bombs at the finish line of the race two weeks ago.

The Wall Street Journal reported yesterday that investigators have found female DNA on at least one of the bombs used in the attacks.

The FBI declined to comment on the matter.

Police said the Tsarnaev brothers set off twin bombs on 15 April that ripped through the crowd watching the race on Boylston Street, killing three people and injuring 264.

Three days later, the brothers led police in a wild car chase through metropolitan Boston, throwing grenades and exchanging gunfire as the officers closed in.

Ms Russell, 24, said through her lawyer last week that she was doing everything she could to assist officials with the investigation.

Her lawyers have not said anything else, but a person familiar with the matter said the legal team has been negotiating how much access authorities will have to their client.

FBI agents have also been seen at Ms Russell's lawyers' office several times since she returned to Rhode Island from Massachusetts on Friday 19 April, after her husband was killed.

Yesterday afternoon she was seen leaving the house with her lawyers and was later seen leaving her lawyers' offices in Providence, Rhode Island.

Ms Russell and Tamerlan Tsarnaev, 26, lived with their young daughter in Cambridge, Massachusetts. Police have said they found bomb material in that apartment.

Her lawyers have said she did not know much about her husband's activities because she spent most of her time working as a health aide near Boston while he was home watching the child.

Two law enforcement officials said both the FBI and local law enforcement agencies are now looking beyond the Boston area to try to identify associates or confederates of the Tsarnaev brothers.

The Wall Street Journal said officials familiar with the case cautioned that there could be multiple explanations for why the DNA of someone other than the two bombing suspects could be on remnants of the exploded devices.

The genetic material could have come, for example, from a shop worker who handled materials used in the bombs or a stray hair that ended up in the bomb, the newspaper said.

Also yesterday, an autopsy on Tamerlan Tsarnaev determined precisely how he died.

The results cannot be made public until the body is claimed, a spokesman for the Massachusetts Medical Examiner said.

Authorities and the public have been waiting to learn whether Tsarnaev died when he was run over by his brother Dzhokhar when the younger Tsarnaev fled in an SUV they had stolen.

Ms Russell would be permitted to claim the body from the medical examiner, a spokesman said.

Dzhokhar Tsarnaev, 19, was captured on 19 April and has been recovering from bullet wounds at a prison medical centre outside Boston.

The brothers' parents, now living in Russia, said on Sunday that they have abandoned initial plans to come to the United States to claim their older son's body and visit their younger son.