The suspects are two black males, Postal Inspection Service spokeswoman Pamela Durkee said. One suspect was about six feet tall and wore a gray or beige hoodie and sweat pants.
The first suspect got into the mail truck and investigators are working to figure out what, if anything, the suspect took.
The first suspect met up with a second suspect and the two left in a Yukon, Durkee said.
The robbery happened just before 3 p.m. Monday.
There is a $50,000 reward for anyone with information that leads to the arrest and conviction of the two suspects.