Complex sentence: In 2001, K-Mart, which sold ammunition to the shooters, announced it would no longer sell handgun ammunition, action encouraged by and documented in the film "Bowling for Columbine".
Split into simpler sentences: In 2001, K-Mart, which had sold ammunition to the shooters, announced it would no longer sell handgun ammunition. This action was encouraged by and documented in the film "Bowling for Columbine".