Winner of two top prizes at Cannes, Gus Van Sant’s fictionalized drama about the Columbine massacre was generated by conversations with the teenage actors about their own lives, and reportedly none of the dialogue was scripted. Perhaps as a result this offers little insight into the motivations of teenage mass murderers, unless one counts such threadbare ideas as a TV documentary about Nazism idly watched by the killers. What interests Van Sant is why no one saw the massacre coming, and his exciting and rigorous structure follows several characters in overlapping trajectories and time frames (a method derived from Bela Tarr’s Satantango) so that we’re constantly noticing details we missed earlier. The effect is riveting and tellingnot always realistic (none of the characters carry cell phones) but often enlightening. 81 min. (JR)