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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.

In another development, Russian officials revealed details about contacts between the older brother and suspected Islamist radicals in the Caucasus, including Internet exchanges that led to concerns by investigators that he was trying to join up with jihadist fighters.

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.”

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