  Using GRB 991216 as a prototype, it is shown 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
(ABM) pulse with inhomogeneities in the interstellar medium (ISM). The initial
phase of such process occurs at a Lorentz factor $\gamma\sim 310$. The crossing
of ISM inhomogeneities of sizes $\Delta R\sim 10^{15}$ cm occurs in a detector
arrival time interval of $\sim 0.4$ s implying an apparent superluminal
behavior of $\sim 10^5c$. The long lasting debate between the validity of the
external shock model vs. the internal shock model for GRBs is solved in favor
of the first.
