Complex sentence: In 1939, the FBI became pre-eminent in the field of domestic intelligence, thanks in large part to changed made by Hoover, such as expanding and combining fingerprint files in the Identification Division, to compile the largest collection of fingerprints to date.
Split into simpler sentences: In 1939, the FBI became pre-eminent in the field of domestic intelligence. Hoover made changes, such as expanding and combining fingerprint files in the Identification Division, to compile the largest collection of fingerprints to date.