By 1934, the Division held more than 7 million fingerprint records — a figure that placed it among the largest fingerprint repositories in the world. These records came from multiple sources: criminal arrests across the country, submitted by local and state law enforcement agencies that cooperated with the federal repository; military enlistment records from the Army and Navy; and voluntary submissions from civilians who registered their prints for identification purposes.
The scale of the repository was itself a law enforcement tool. It allowed an investigator who recovered fingerprints at a crime scene to submit those prints and check them against a database large enough that an identified criminal — someone who had been arrested before — was likely to be in the files. The system worked on the principle of prior contact with law enforcement: a person who had been fingerprinted once was permanently in the record, and any subsequent identification attempt could reference that prior record.
Dillinger’s record was in the files from his 1924 conviction. Every subsequent arrest had added more impressions, confirming the record and providing additional comparison material. When the postmortem prints arrived from Chicago, the examiner did not have to search the entire database; they went directly to the known Dillinger record and compared.
The comparison took time and care — the alteration damage required careful examination of the peripheral areas — but it did not require any technique beyond what fingerprint examiners were doing routinely. The case was extraordinary in its public significance; it was not extraordinary in its forensic demands.