According to the caption in the splash panel, only one person knows that Joe Higgins is the Shield, and that is the head of the FBI himself, J. Edgar Hoover! The Shield is a G-Man, who works for the FBI. They must’ve had a superhero division in the FBI at one time. Plastic Man worked for them, too.
I showed the origin of the Shield back in 2009, and you can see it by clicking on the link below.
From Pep Comics #3 (1940). Art by Irv Novick.
An origin of the Shield was featured in Shield-Wizard Comics. Just click on the thumbnail.
