The sealed warrant is probably the same as our 'secret indictment', which has to be obtained via a grand jury. That means the officer (probably a detective) goes before the grand jury if he thinks he has enough evidence to file for a warrant.
Then why don't they say so?
It would go a long way toward satisfying us "cop-haters" if LEAs didn't act like nothing they did was anybody's business but their own.
Peace.