We did some digging and discovered the football carrier is Ricardo Turner ... one of Trump's military aides. He became an international man of mystery Sunday when a Mar-a-Lago guest posted a photo with him saying ... "This is Rick...He carries the 'football.'"

The nuclear football is the briefcase for the hardware used by POTUS to authorize a nuclear attack while away from the fixed command centers, which means Ricardo's the Man.

Trump promoted Ricardo from Major to Lieutenant Colonel on Monday in an Oval Office ceremony ... according to White House Spokesman Sean Spicer.

Aides like Ricardo provide direct support to the prez and serve as emergency action officers, but not all of them get to carry the football.