We investigate the interaction of ultrashort laser filaments with individual 90 µm ice particles, representative of cirrus particles. The ice particles fragment under laser illumination. By monitoring the evolution of the corresponding ice/vapour system at up to 140,000 frames per second over 30 ms, we conclude that a shockwave vaporization supersaturates the neighbouring region relative to ice, allowing the nucleation and growth of new ice particles, supported by laser-induced plasma photochemistry. This process constitutes the first direct observation of filament-induced secondary ice multiplication (FISIM), a process that strongly modifies the particle size distribution, hence the albedo, of typical cirrus clouds.