Hoover maintained separate, master national blacklists—such as the Security Index—which tracked tens of thousands of citizens deemed "subversive" or political dissidents for immediate detention in the event of a national emergency. He managed his master national blacklists through a fluid, multi-tiered indexing framework that evolved over five decades. These blacklists were not mere static documents; they were part of a highly coordinated operational pipeline designed for the mass roundup and indefinite detention of American citizens during a perceived national emergency.

To add or manage a person on a master blacklist, Hoover’s Bureau followed a strict administrative lifecycle:

The Dossier Trigger: When an individual was flagged via covert programs like COINTELPRO, agents opened an investigative file.

The Index Card Core: If the person was deemed a threat, a dedicated index card was generated. These cards contained the person's name, aliases, address, physical description, occupation, and a specific "detention rationale.

"The Geographic Apportionment: Cards were duplicated and cross-filed. One went into the master archive at FBI Headquarters in Washington, D.C., and another went into the local FBI Field Office responsible for the geographic area where the target lived.

The Arrest Portfolio: For top-tier targets, field offices maintained ready-to-go arrest portfolios. If Hoover or the President gave the command, field agents could immediately seize the individual without needing to waste time researching where they were or why they were being detained.

The Fluid Evolution of Sub-Lists

Hoover managed the master blacklist by segmenting it into constantly shifting sub-indexes based on perceived ideological threats, which allowed him to scale the operation up or down

As the lists ballooned to over 20,000 active high-priority targets (and over 10 million Americans cross-indexed in general domestic files), Hoover modernized his management using early technology. The Bureau adopted mechanical punch-card sorting systems. This allowed clerical staff to instantly filter the master blacklist by city, profession, or political affiliation, providing Hoover with rapid statistical snapshots of domestic dissent to present during congressional budget hearings or White House briefings.