The older suspect in the Boston Marathon bombings spoke to his mother about "jihad" in a 2011 phone call secretly recorded by Russian officials, CBS News reported on Saturday.
U.S. authorities learned of the wiretapped discussion between Tamerlan Tsarnaev, one of two ethnic Chechen brothers suspected of carrying out the April 15 blasts in Boston, and Zubeidat Tsarnaeva within the last few days, CBS said.
It provided no other details.
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, 26, died in a shootout with police in Watertown, Massachusetts, last week.
His brother, 19-year-old Dzhokhar Tsarnaev, was captured and has been charged with using a weapon of mass destruction and malicious destruction of property in connection with the Boston attack, which killed three people and wounded 264.
Tsarnaeva and the suspects' father told reporters on Thursday in Makhachkala, the capital of Russia's Dagestan region, that they believed their surviving son was innocent.
Attention has turned to whether U.S. officials missed signs that Tamerlan Tsarnaev may have posed a security threat, including a warning from Russia that he might be an Islamic militant.
The FBI interviewed him in 2011 but did not find enough cause to continue an investigation.
Tamerlan Tsarnaev's name was listed on the U.S. government's highly classified central database of people it views as potential threats, sources close to the bombing investigation have said.
Law enforcement authorities do not closely monitor the list, which includes about 500,000 people.