Summary: The development of a three-dimensional nonlinear wave packet in axisymmetric boundary layer is analyzed numerically within the framework of asymptotic theory of free interaction. It is found that rapid breakdown of the wave packet occurs as a result of interaction between harmonics of the lowest nonaxisymmetric mode, which leads to a faster buildup of the frequency-wavenumber spectrum. The buildup of the spectrum proceeds through phase coupling, which leads to the occurrence of powerful spiky bursts characteristic of the \(K\)-regime of laminar-turbulent transition.