The intercepted calls were first reported by The Associated Press on Saturday. Both calls involved Zubeidat Tsarnaeva, the mother of Tamerlan and his younger brother, Dzhokhar. The authorities believe that the brothers set off two homemade bombs at the marathon’s finish line. Tamerlan, 26, died after being shot by the police and run over with a vehicle driven by his fleeing brother; Dzhokhar, 19, was shot but is recovering in a prison hospital.

One call was between Tamerlan Tsarnaev and his mother; the other was between Ms. Tsarnaeva and another person in Russia who is under F.B.I. investigation in an unrelated matter. The calls mentioned jihad — a central concept in Islam that sometimes can mean holy war — but no specific attack plans, the official said…

Had the F.B.I. and C.I.A. known the details behind the warning, it is possible that a full investigation might have been ordered. “Would we have gone farther?” one law enforcement official said. “Maybe, maybe not.”