The SA position is just that, pretty self-explanatory. The investigator position is the old inspector position, conducting the compliance inspections of the FFLs and FELs, the regulatory side of things. This is an 1801, non-firearm carrying general investigator position. The Explosive Enforcement Officer position is an explosives specific position. They are trained solely for that specialization. They get hands on with the explosives, both before and after detonation, they also get render safe training. When comes to explosives, the SAs primarily investigate post-blast or post-render safe. The SAs don't get hands on with rendering safe, they call in the EEOs or the local/state bomb squads.
Go to the ATF website to get more specific info.
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