[en] In this paper we investigate the sawtooth crash in the ASDEX Upgrade tokamak and present evidence supporting the hypothesis of stochastization of magnetic field lines during the crash. We demonstrate on the basis of soft x-ray and electron cyclotron emission measurements that during the pre-crash phase the quasiperiodic transition to chaos occurs. Magnetohydrodynamic oscillations with two frequencies develop before the crash. Consistent with the most energetically favourable transition from quasiperiodicity to chaos, their frequency ratio is close to the conjugate golden ratio G=f₂/f₁=(√5-1)/2∼0.618. We think that the sawtooth crash has a universal stochastic character and it would be worthwhile to search for these transition signatures in other tokamaks