How does Federal "seizing" of State purchased tests work?

Today, there was this story about Gov Hogan stationing National Guard and State police to guard coronavirus tests at a “secret location” to protect against Federal seizure.

Who from the Federal government is going to show up and demand that the state hand over the tests? FBI? Federal Marshals? Who would just give up testing kits to a random person who says “I’m from the government, give me all the tests you just bought!”?

Rachel Maddow had a story about the FBI charging some people who defrauded the California public service union about 60 million medical masks for sale. Why? Because when the feds started looking into how they could seize masks bought by Californians for Californians, they found it was a fraud.

IIRC the federal government under the emergency powers act has from what I understand sent federal marshals (no doubt supervising rented trucks and hired drivers) to take possession of medical supplies under their emergency powers. Allegedly this is to build the national stockpile which it can then distribute to any state or organization with a pressing need, without favoritism. (/s)

News stories that donors with connections have treated this also as a private shopping opportunity are not proven.