I’m not sure it is illegal, but I’m not a lawyer. FBI asked someone with a key to let them in, and they complied.
If the police knock on your door and ask to come in, and you let them in, they haven’t done anything illegal with their entry.
Now the man could have felt pressured by the FBI and thus it wasn’t totally voluntary. We don’t have that info.
Without a warrant before the search, it is an illegal search.
Would it make a difference if the person who let them in was not an authorized representative of the owner? What if they wanted to enter a house and a neighbor let them in?
The person with the key was an 3-demployee .... not authorized
Just because FBI agents obtained a key, that did not give them the authority to enter. This move was illegal and the fruit of the illegal entry should not be allowed as evidence.