Are you in the US and are you familiar with things like vehicle registration?
Are you unable to comprehend that if a vehicle does not have a federally-issued license plate, then it still requires a plate to be legal on the road... in which case, federal vehicles can use state plates. As quoted many times above, including my original comment.
Allowing law enforcement vehicles of any kind to go plateless would make it very simple for anyone wishing not to be tracked to go plateless. "Oh look, there's the FBI in an unmarked '95 Corsica" ... wait, no, driving without plates is not legal, so that '95 Corsica is probably just a driver without plates. Pull them over.
Driving without plates will get you pulled over in any state in the US.
Again, help me understand why you need another source to connect this information? Have you tried to make sense of the link you posted? I'm not your tutor, so I'm not sure what kind of help is needed to make this clear to you: you cannot drive on any public road in any state without a license plate.