These photos give us a rare look inside the FBI’s overflow filing system. The FBI’s archive collected over 23 million cards and 10 million fingerprint records, and housed them in the Washington, D.C. Armory, during World War II. They added around 400,000 new cards every month.

Let's take a closer look.

GEORGE SKADDING/THE LIFE PICTURE COLLECTION/GETTY IMAGES

The federal government invested huge resources to investigate potential defectors and spies during this time, .

GEORGE SKADDING/THE LIFE PICTURE COLLECTION/GETTY IMAGES

By the end of 1943, the FBI employed around 13,000 people.

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Since 1924, the FBI has been the single U.S. repository for fingerprints.

GEORGE SKADDING/THE LIFE PICTURE COLLECTION/GETTY IMAGES

GEORGE SKADDING/THE LIFE PICTURE COLLECTION/GETTY IMAGES

GEORGE SKADDING/THE LIFE PICTURE COLLECTION/GETTY IMAGES

GEORGE SKADDING/THE LIFE PICTURE COLLECTION/GETTY IMAGES

GEORGE SKADDING/THE LIFE PICTURE COLLECTION/GETTY IMAGES