More details continue to emerge about the two suspects in the Boston Marathon bombings.
U.S. media report Tamerlan Tsarnaev spoke to his mother, Zubeidat Tsarnaeva, about "jihad."
The reference came in a 2011 phone call secretly recorded by Russian offiicals.
CNN quoted U.S. Attorney General Eric Holder as saying that the matter was "ongoing" and that he could not comment on it.
Jihad can refer to a holy war waged on behalf of Islam as a religious duty, or to a Muslim's personal struggle in devotion to the faith.
Tamerlan Tsarnaev was killed in a shootout with police.
His younger brother, Dzhokhar, is now facing criminal charges, including using a weapon of mass destruction in connection with the Boston attack, which killed three people and wounded 264.
Based on reporting by Reuters and AP