(CNN) -- President Barack Obama is defending the FBI's handling of a 2011 request by Russian authorities to investigate Tamerlan Tsarnaev, one of the two brothers accused of bombing the Boston Marathon.
Some critics have questioned whether the agency did enough to investigate Russian reports of growing extremism in Tsarnaev, who died April 19 after a firefight with police, four days after the bombings.
But at a White House news conference Tuesday Obama said "it's not as if the FBI did nothing."
"They not only investigated the older brother, they interviewed the older brother," the president said. "They concluded that there were no signs that he was engaging in extremist activity."
Meanwhile, investigators were 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.
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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.
Photos: Galleries from the attack and aftermath
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.
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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.
Also Monday, news emerged that federal agents are looking into possible links between Tamerlan Tsarnaev and a Canadian jihadist killed by Russian troops in 2012, a source being briefed on the investigation said.
William Plotnikov and six others died in a firefight with Russian forces in the southwestern republic of Dagestan in July 2012 -- while Tsarnaev was visiting the region, the source said. The 23-year-old Plotnikov was born in Russia, but his family moved to Canada when he was a teenager.
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Tsarnaev flew out of Dagestan two days after Plotnikov's body was prepared for burial, according to the source. Investigators are looking into the possibility he left because of Plotnikov's death, the source said.
Like Tamerlan Tsarnaev, Plotnikov was once a boxer.
Investigators also are looking into whether Tsarnaev had any contact with another militant named Mahmoud Mansur Nidal, who was killed by Russian forces in May 2012 during a gun battle in Dagestan's capital, the source said.
The FBI conducted an investigation -- including an interview with Tamerlan Tsarnaev -- after Russia expressed concerns in 2011 about his possible radicalization.
The Russians also raised questions about Tsarnaev's mother, Zubeidat Tsarnaev, according to several sources. Her name was subsequently added to a terrorism database along with her son's, an intelligence official said last week.
Russian authorities intercepted a phone call in early 2011 from one of the Tsarnaev brothers in the United States to their mother in Dagestan that included a vague discussion of jihad, an official with knowledge of the investigation told CNN over the weekend.
That information didn't make its way to the FBI, and the agency has said its investigators found nothing in 2011 to justify further investigation. The case was closed after several months.
On Monday, Zubeidat Tsarnaev told CNN's Nick Paton Walsh that she plans to come to the United States if she can see her son, despite pending shoplifting charges against her in Massachusetts. She has expressed doubts about the bombing and her sons' role in it.
Also Monday, a U.S. government official told CNN that FBI agents have interviewed the man identified as "Misha," an elusive figure whose name has surfaced in the Boston bombing investigation.
Investigators spoke with the man in Rhode Island after reports surfaced suggesting that members of the suspected bombers' family blame a "Misha" for radicalizing Tamerlan Tsarnaev.
The man, whose real name is Mikhail Allakhverdov, denies ever encouraging a violent take on Islam and says he was not Tamerlan's teacher, according to a New York Review of Books writer who says he interviewed the man.
Allakhverdov insisted he had "nothing to do with radicalization," and said he was cooperating with the FBI, reporter Christian Caryl told CNN on Monday.
CNN has made repeated efforts to speak with Allakhverdov, but has so far been unsuccessful.