Bob, if you do some digging on that question, you'll find they are not "federalized," which has been an issue in the past. So, they essentially fall under state law in terms of certifications and regulation. It's been one of the reasons the FPS has been so problematic with so many longstandng issues with vetting these private contractors and ensuring proper training and no criminal background, etc.

Some evidence:

EXAMINING THE FEDERAL PROTECTIVE SERVICE:ARE FEDERAL FACILITIES SECURE?(113–71)

HEARING BEFORE THE SUBCOMMITTEE ON ECONOMIC DEVELOPMENT, PUBLIC BUILDINGS, AND EMERGENCY MANAGEMENT OF THE COMMITTEE ON TRANSPORTATION AND INFRASTRUCTURE HOUSE OF REPRESENTATIVES ONE HUNDRED THIRTEENTH CONGRESS SECOND SESSION MAY 21, 2014

Ms. NORTON. Thank you very much, Mr. Chairman. I very muchappreciate this hearing. There have been chronic problems at FPS. And I would like to look at the difference between FPS officers [federal agents] and a contract guard [private contractors] so that we understand who is really guarding these buildings. On page 2 of your testimony, you describe the law enforcement authority of FPS officers, specific police powers, including enforcing Federal laws and regulations, carrying firearms, et cetera.

****Then, of course, on page 5 of your testimony you distinguish these officers from the contract guards who—and this is very important, I think,to just lay right here on the record, it is in your testimony—the PSOs [contract FPS guards] rely on private person laws, such as citizen arrest laws. So that means that they can do no more than I can do [as a private citizen] in a Federal building. I mean, isn’t that technically correct?

Mr. PATTERSON. Yes, ma’am. They are governed by the State law as to the extent of their authority.

..... Now, assuming they are assisting with crowd control in these "secret police" units as they are authorized to do as part of their job description, then it seems they are doing so under the particular state's jurisidction and regulation, not with federal policing authority. Whether Trump and Barr have conferred some sudden federal power on them is not clear, though that would seem to be a stretch of their own power. These are issues that need further investigation.

Another source, a trade publication covering federal agencies, puts it this way in layman's terms:

FPS says it tells its contract guards that if they encounter an active shooter, they should engage him or her, with deadly force if necessary. But the agency agreed there’s a lack of clarity about those contractors’ legal authority.

Since security guards are not federal law enforcement officers, their police powers generally are governed by the state and local laws at the location of a given federal facility. And in most places, that means they have no more legal authority than an average citizen.