non-interference with pending administrative proceedings

as long as the proceedings are in fieri, the courts will not interfere with the hearing and disposition of matters before the Departments... To hold that the writ could issue either before or after an administrative ruling would make the despatch of business in the Departments wait on the decisions of the courts, and not only lead to consequences of the most manifest inconvenience, but would be an invasion of the executive by the judicial branch of the government.