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.”
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.
Both U.S. and Russian authorities are trying to learn more about Tamerlan Tsarnaev’s activities while staying in Russia from January to July of 2012. President Barack Obama and Russian President Vladimir Putin spoke by telephone Monday and agreed to boost antiterror cooperation, officials on both sides said.
Two Russian government officials said Tamerlan Tsarnaev exchanged notes over the Internet with William Plotnikov, a boxer who moved with his parents from Russia to Canada before joining militants in the North Caucasus. And they said Mr. Tsarnaev met several times in early 2012 with Mansur Makhmud Nidal, an alleged militant from the Russian province of Dagestan and suspected jihadist recruiter. The meetings happened in a mosque in Dagestan’s capital of Makhachkala known for its adherence to a puritanical strain of Islam, they said.
The Tsarnaevs’ parents have denied their sons were involved in terrorism.
Mr. Nidal died in a firefight in Makhachkala last May after a five-hour standoff that ended with him throwing a grenade at police officials, according to Russian authorities. Mr. Plotnikov died two months later, in mid-July, during a raid in the hinterland of Utamysh, a village southwest of Makhachkala.
Mr. Tsarnaev was in the region at the time of both raids but left Russia for the U.S. three days after the second one. He left before picking up a Russian passport that had been prepared for him at the local migration office in Dagestan’s capital.
“He intended to join the fighters, but he lost his contacts,” one of the Russian officials said. “In the end he picked an easier enemy in Boston.”
The Russian officials said it was unclear whether Mr. Tsarnaev and Mr. Plotnikov ever met in person. The parents of the two boxers say they don’t recall any interaction between the two. There are no indications that the two men ever boxed together, and Russian officials haven’t yet produced any of the online communication that they say exists.
A U.S. official said Russian authorities haven’t shared any information about Mr. Tsarnaev related to contact with any suspected militants during his trip last year to Dagestan. Several U.S. officials, however, said they believe Russia has more information about Mr. Tsarnaev that hasn’t been handed over.