An official, who asked not to be identified because the investigation was ongoing, said there could be multiple explanations, including a store clerk who handled materials that Tamerlan Tsarnaev and his younger brother, Dzhokhar, used to make the bombs, or perhaps genetic material that unwittingly ended up on the explosive devices, perhaps from a marathon bystander.
The Wall Street Journal first reported the find.
Earlier Monday, FBI agents went to a Rhode Island home to collect a DNA sample from Katherine Russell Tsarnaeva, the widow of Tamerlan Tsarnaev, who died in a shootout with police April 19. She has been staying with her parents since the bombings.
The sample will be compared with the genetic material recovered.
"The FBI is there as part of our ongoing investigation, but we aren't permitted to discuss specific aspects of our case," said FBI spokesman Special Agent Jason Pack.
Agents left her parents' home in North Kingstown with several bags. Russell Tsarnaeva left with her attorneys through a separate door, the Associated Press reported.
Last week, attorney Amato DeLuca said she was "doing everything she can" to assist the investigation.
Russell Tsarnaeva, a Boston college dropout, converted from Christianity to Islam before marrying Tamerlan Tsarnaev three years ago and later giving birth to a daughter. She supported the family by working as a home health aide.
Her attorneys said previously that Russell and her family were shocked when the Tsarnaev brothers emerged as suspects.
Dzhokhar Tsarnaev, 19, has been charged with using a weapon of mass destruction and is being held in a U.S. prison medical facility at Fort Devens, about 40 miles west of Boston. He was wounded either during the gunbattle that killed his brother or during the manhunt before his capture the night of April 19.
Monday, a federal judge approved the addition of a prominent death-penalty lawyer, Judy Clarke, of San Diego, to Tsarnaev's legal team. A request for a second capital-punishment expert was denied.
Clarke assisted Jared Loughner, who is serving a life sentence for the Tucson, Ariz., shootings that killed six and wounded 13, including former congresswoman Gabby Giffords. She also defended Ted Kaczynski, the Unabomber, and Susan Smith, who drowned her two young sons in 1994. They, too, are imprisoned for life.