(ONLINE.WSJ) — 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.

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