Complex sentence: He was then the head of the homicide division from 1939 until his retirement in 1944 when he became the Chief of Police at Kennywood, an amusement park in neighboring West Mifflin, Pennsylvania.
Split into simpler sentences: He was then the head of the homicide division from 1939 until his retirement in 1944. He then became the Chief of Police at Kennywood, an amusement park in neighboring West Mifflin, Pennsylvania.