We analyze by Monte Carlo simulations and analytically the spin dynamics of a two-dimensional electron gas (2DEG) interacting with short-range scatterers in nonquantizing magnetic fields. It is shown that the spin dynamics is non-Markovian with the exponential spin relaxation followed by an oscillating tail due to the electrons residing on closed trajectories. The tail relaxes on a long time scale due to an additional smooth random potential and inelastic processes. The developed analytical theory and Monte-Carlo simulations are in quantitative agreement with each other.