Evolution of three-dimensional disturbances in an accelerating swept laminar boundary layer is investigated by direct numerical simulations and linear parabolized stability equations (PSE). First, so-called controlled transition simulations were carried out where a single unsteady secondary instability wave is added to the baseflow distorted by a stationary primary crossflow vortex, and the nature of secondary instability regimes was identified. Next, the development of a three-dimensional wave packet was investigated, where the wave packet is generated by a localized short-duration pulse disturbance. DNS data support the results obtained from a PSE analysis, namely that the underlying baseflow is unstable with respect to both stationary and traveling crossflow modes with a wide range of frequencies and spanwise wavenumbers, whereby traveling crossflow modes are reaching the largest maximum amplitudes. Nonlinear mechanisms eventually trigger the transition process, as the wave packet breakdown to turbulent patches.