Investigators have found female DNA on at least one of the bombs used in the Boston Marathon attacks, though they haven't determined whose DNA it is or whether its presence means a woman helped the two brothers suspected in the bombings, according to U.S. officials briefed on the probe.

Speaking Monday about the DNA discovery, the U.S. officials cautioned that there could be multiple explanations for why genetic material from someone other than the two bombing suspects -- Tamerlan Tsarnaev and his younger brother, Dzhokhar -- could have been found on remnants of the exploded devices. It could have come, for example, from a store clerk who handled materials used in the bombs or a stray hair that ended up in the bomb.

Monday, Federal Bureau of Investigation agents visited the Rhode Island home of the parents of Katherine Russell, the widow of Tamerlan Tsarnaev. He died after a shootout with police four days after the April 15 bombings.

'The FBI is there as part of our ongoing investigation, but we aren't permitted to discuss specific aspects of the case,' said FBI spokesman Jason Pack.

Ms. Russell has been staying with her parents since the bombings, and FBI agents have been seen posted outside the house since her late husband was identified as one of the bombers. Her lawyer has said she is 'doing everything she can to assist with the investigation.'

One official familiar with the case said agents went to the house Monday to collect a DNA sample from Ms. Russell, the culmination of days of negotiations. FBI officials also have been negotiating with Ms. Russell's attorney in recent days to gain fuller access to question her, the officials familiar with the case said. The officials said the DNA request was needed to determine whether it matched the DNA found on the bomb remnants.

Ms. Russell is one of as many as a half-dozen people in whom investigators are interested as they seek to determine if the brothers had any help in the bomb attack or the days afterward, the officials said. Ms. Russell's lawyer didn't immediately respond to a request for comment about the latest developments.

Investigators have no evidence that any of the associates of the brothers had knowledge of the bombing plot, and they believe that if anyone helped to dispose of evidence -- a point that remains uncertain -- it may have been unwitting, according to officials briefed on the investigation.