Who the f*ck are "United States Police" and why are they roaming around my neck of the woods today??

Driving in to work, I was following behind two police cars. Upon closer inspection, they had no jurisdiction markings at all. Just white cars, lotsa emergency lights, blue striping, the word "POLICE" and an ID number on the trunk (5505 and 5515), and a gold shield that said "UNITED STATES". As we approached Tyson's Corner VA, I saw another one driving by in the opposite direction. I've driven this route daily for the last 4+ years and this is the first I've seen of them...but three in at a time?

Am I the only one creeped out by mystery "United States Police"? Didn't realize we had a national police force.