[en] Various reaction mechanisms proposed to explain the pion production at 'subthreshold' energies (below 290 MeV/A) are examined. They range from the nucleon-nucleon single collision mechanism to a co-operative multi-nucleon process. With a shell model prescription for the initial state energies the single collision picture can not explain the data. The participation of many nucleons in the pion production process appears to be necessary. We present a statistical model where the co-operative action of several of the target and projectile nucleons in the pion production process is invoked. We also consider the formation of the fragments in the final channel alongside the produced pion. Calculations performed within the model provide a good overall description of the experimental data over a wide range of beam energies and masses of the participating nuclei. Fragment formation in the final channel is seen to be vital to understand the experimental data within our model. (orig.)