Using GRB 991216 as a prototype, we show that the intensity substructures observed in what is generally called the "prompt emission" in gamma-ray bursts (GRBs) do originate in the collision between the accelerated baryonic matter pulse with inhomogeneities in the interstellar medium (ISM). The initial phase of such process occurs at a Lorentz factor γ～ 310. The crossing of ISM inhomogeneities of sizes ΔR ～ 10~(15) cm occurs in a detector arrival time interval of ～ 0.4 s, implying an apparent superluminal behavior of ～10~5c. The long-lasting debate between the validity of the external shock model versus the internal shock model for GRBs is solved in favor of the first.
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