The official said it remains unclear what kind of detonator was used for the bomb, which investigators say was packed into a panel truck and killed at least seven people when it went off.
The official was speaking on condition of anonymity because the information has not been formally released.
A 4,000 pound (1,815 kilogram) fertilizer-and-fuel-oil bomb detonated in front of a federal building in Oklahoma City in 1995, killing 168 people.