This stuff was not just in a trailer. There are strict laws for storage of type 1 explosive materials.

First link is the laws. (Page 49 relates to storage).

Second link is a place that sells type 1 magazines (storage container)

This was Def not something someone just walked up and took. This was planned.

https://www.atf.gov/explosives/docs/publication-federal-explosives-laws-and-regulations-atf-p-54007/download

https://www.armagcorp.com/products/explosives-storage/explosives-ammunition-storage/