Good question! The immediate answer is that while there was in fact a problem with police brutality in the United States before World War 2, due to the rise in organized crime.

In 1919, after the establishment of the Volstead Act (and, by association, the prohibition of alcohol), crime rates in America skyrocketed. The biggest difference between crime in the Prohibition Era and crime before the Prohibition Era was that now, the criminals were becoming organized. Rather than a murder being committed because of a dispute or a cheating spouse, it was now being done for the advancement of a criminal syndicate (see The Saint Valentine's Day Massacre. As a result, the police were forced to begin expanding their efforts, since cases were much less open-and-shut. More complicated crimes, paired with the pursuit of criminal leaders like Al Capone, lead to an increase in police work and urgency, and with it came an increase in police brutality. By the late 1920s, the police brutality became enough of a problem in the eyes of the public that it was taken to the Federal government, and the National Committee on Law Observation and Enforcement (colloquially known as the Wickersham Commission) was formed to investigate in 1929 by President Hoover. In 1931, the Wickersham Commission published the "Report on Lawlessness in Law Enforcement", which acknowledged the widespread use of brutal tactics by the police, and proceeded to condemn them. Five years later, in 1936, the landmark case Brown vs. Mississippi was brought before the Supreme Court, which firmly established a legal precedent for the due process clauses of the Fifth and Fourteenth Amendments.