BOSTON, M.A. (CNN)--Investigators are trying to find the source of a woman's DNA found on a fragment of one of the pressure cooker bombs used in the bombings, law enforcement sources told CNN.
But, they cautioned, that doesn't necessarily mean a woman might have conspired with brothers Tamerlan and Dzhokhar Tsarnaev, who authorities say were behind the April 15 attack that killed three and injured more than 260. The brothers also allegedly killed a police officer.
Twenty people remained hospitalized Tuesday, according to a CNN tally.
One of the sources noted Monday that the DNA on the bomb component could have come from any woman who touched any of the items used to make the bomb.
The FBI did take DNA samples at the Rhode Island family home of Tsarnaev's widow, Katherine Russell, on Monday in an effort to see if the genetic material belonged to Russell or the couple's 3-year-old daughter.
But one of the officials said that even if Russell's DNA matches that from the bomb fragment, it doesn't necessarily mean she participated in the bomb's construction.
Through her attorney, Russell has denied any knowledge of her husband's involvement in the bombings and has said she is cooperating with the investigation.
The DNA could also be from one of the victims, Lawrence Kobilinsky, a DNA expert at the John Jay College of Criminal Justice in New York, told CNN's Erin Burnett on Monday.
The development adds yet another thread to an investigation that has stretched from Boston to Rhode Island to Russia as authorities try to unravel why the Tsarnaev brothers allegedly targeted the marathon, and if anyone helped them along the way.
The elder Tsarnaev died April 19 after a firefight with police. The Massachusetts Office of the Chief Medical Examiner said Tuesday that its investigators have determined Tsarnaev's cause of death, but can't release that information until his remains have been retrieved and a death certificate filed.
His 19-year-old brother Dzhokhar is being held at a federal Bureau of Prisons medical center in Devens, Maryland, on a charge of using a weapon of mass destruction. He faces a possible death penalty if convicted.
On Monday, a federal judge appointed prominent defense lawyer Judy Clarke to represent him.
Clarke has represented Ted Kaczynski, the Unabomber; Eric Rudolph, the 1996 Atlanta Olympics bomber, and Jared Lee Loughner, who pleaded guilty in the Tucson, Arizona, shooting that killed six and left then-U.S. Rep. Gabrielle Giffords seriously wounded.