© ALAMYSchriever Air Force Base
The unnamed man is said to be carrying a handgun and is a member of the 50th Security Forces Squadron, a unit responsible for guarding Schriever Air Force base.
As yet no shots have been fired and the man has not taken any hostages but security personnel are surrounding the employment process building where he is based.
The building has been evacuated and "special control measures" are in place at the base, which is just south of Denver.
"The security of Schriever personnel and their families is paramount. We are taking every precaution to ensure their safety," Colonel James P. Ross, 50th Space Wing commander said in a statement.
The 50th Security Squadron is tasked with security at the 6,000-strong base and it is likely that the armed man's comrades are involved in the siege.
A negotiator from the El Paso County Sherriff's office has been sent to help with the situation.
"it is likely that the armed man's comrades are involved in the siege."
"I" would definitely like to know.