Basically, there were 100 bombs found that day, and Klebold and Harris didnt' bring them in alone. And the fact that an eyewitness said he saw two cars, with seven kids, at the school early that morning confirms there were accessories to the massacre.
John Stone, the Sheriff of Jefferson county at the time, believed there were others involved, but he was constantly blocked by an FBI agent named Dwayne Fuselier. To make matters worse, Fuselier had two sons in the Trench Coat Mafia, and the FBI saw no conflict of interest.