FBI documentation regarding Columbine’s aftermath, in which can be seen what the major focuses of their investigation were. Interesting to note is that there were additional threats regarding a potential bombing on April 26, 1999, that they were investigating at the time. There are almost no indications of this in media reports from that period, however, so it’s likely that this threat was kept under wraps.

However, the choice of the term “secondary explosive devices” is rather odd. At a site like Columbine, the bombs in their cars would have been categorised as secondary explosive devices. Secondary explosive devices are primarily used on-scene of an attack to injure/kill emergency responders. They’re not the main focus of the attack, but exist to further confuse and harm people on-scene. Whatever was designed to occur on April 26, these secondary explosive devices mentioned might only be a small part of it.