Investigators have found female DNA on a fragment from one of the two bombs used in the attack on the Boston Marathon two weeks ago, according to a US official briefed on the probe.
The genetic material may have come from a number of sources and its discovery does not necessarily mean that additional people were involved beyond the two brothers suspected in the bombing, said the official.
The DNA find emerged as authorities continue to investigate whether the suspects, Tamerlan and Dzhokhar Tsarnaev, had help in planning or carrying out the April 15 attack. So far, according to a second US official, there is no evidence of that.
Among those under scrutiny is Tamerlan's wife, Katherine Russell Tsarnaev. Agents from the Federal Bureau of Investigation visited her parents' home in North Kingstown, in the state of Rhode Island, on Monday and took a DNA sample from her.
Amato DeLuca, a lawyer for Katherine Tsarnaev, said that she was continuing to "assist in the investigation in any way she can". Last week, he said that she knew nothing about the attacks beforehand and "was as shocked as anybody" when they happened.
An FBI spokesman declined to say whether Katherine Tsarnaev was co-operating voluntarily and said that that the bureau could not discuss specific aspects of the case.
Tamerlan Tsarnaev, 26, was killed in a police shoot-out April 19. He and his brother are suspected of setting off the bombs at the Boston Marathon finishing line that killed three people and wounded 260.
Dzhokhar Tsarnaev, 19, was apprehended later that day after a manhunt that paralysed the Boston metropolitan area. He was found hiding in a boat in the backyard of a house in Watertown, a city suburb. He was taken to hospital with gunshot wounds.
The younger Tsarnaev, now being held in a prison west of Boston for inmates needing medical care, is charged with using and conspiring to use a weapon of mass destruction.