“There is no provision for a Federal police agency in the Constitution.”
You are 100% correct.
Normal police and criminal powers reside with the states.
Armed Federal officers are nothing more than a standing mercenary army stationed among us...120,000 of them at last count.
Other than US Marshals, Postal Police, and maybe Customs arming these thugs and thuglets is a very recent development.
The Secret Service is not Constitutionally specified/permitted other than as customs/currency agents for Treasury/tariffs.....Lincoln used Pinkerton’s.
Their body guard duties I guess would be extensions of the DC Federal/Capitol police authority over Federal lands and security.
Yet more checks/balances torched for convenience and now accepted as normal.