Complex sentence: The cases of these aliens, whether being considered for internment or under internment, were managed by the Enemy Alien Registration Section of the Department of Justice, headed beginning in December 1917 by J. Edgar Hoover, then not yet 23 years old.
Split into simpler sentences: The cases of these aliens, whether being considered for internment or under internment, were managed by the Enemy Alien Registration Section of the Department of Justice. From December 1917 this section was headed by J. Edgar Hoover, then not yet 23 years old.