Mixed quantum-classical dynamics simulations at the multireference configuration interaction (MR-CIS) level were performed for 9H-adenine in order to understand its ultrafast nonradiative decay process. Dynamics simulations were also performed for the model system 6-aminopyrimidine. MR-CIS and complete active space perturbation theory (CASPT2) have been employed to characterize a large variety of qualitatively different conical intersections, the branches of the crossing seam connecting them, and the reaction paths from the Franck-Condon region for 9H-adenine. The results show a two-step process consisting of ultrashort deactivation from S-3 to S-1 and a longer exponential decay step corresponding to the conversion from S-1 to S-0.