The FBI is generally more constrained. I guarantee in those papers Calhoun had was a search warrant. You also have to cross all your t's.
Back when my friend was unjustly charged with a murder, he showed me a search warrant they used to search a house he owned. I looked at it and said it wasn't valid. They had gone to the judge at the end of the day and his clerk had already changed the date on the calendar, so the warrant was for the next day. They didn't find anything anyway, but the search was still illegal.