It seems that in 1928, President Hoover appointed what was known as the Wickersham Commission to look into the effects of Prohibition. One section of the commission's report, which was delivered in 1931, drew attention to the issue of extreme police brutality during interrogations (which had often been used against political dissidents as well as common criminals.)
Most of the information that I'm finding on this is showing up only in Google Books, snippets of academic papers, and images of old newspapers, none of which can be copy-and-pasted. For example, there's an image of a newspaper story from 1931 at http://news.google.com/newspapers?nid=1338&dat=19310810&id=Kt1YAAAAIBAJ&sjid=3vQDAAAAIBAJ&pg=7033,2053923 which discusses the commission's findings about third degree methods, coerced confessions, and "lawlessness in law enforcement." It also quotes an acting district attorney as calling the report "absurd" and asking, "What are we to do, give our baby killers ice cream sodas?"
There's also a discussion of some of this in a Google Book preview at http://books.google.com/books?id=JzZFZPrdn-oC&pg=PA81&lpg=PA81&dq=%22wickersham+report%22+police+brutality&source=bl&ots=dU93DWTQkm&sig=dy68d-Okd1eBNKb_hEReuZRPfIg&hl=en&sa=X&ei=QUKlT_jLMIys0AGupPWlBQ&ved=0CHMQ6AEwCQ#v=onepage&q=%22wickersham%20report%22%20police%20brutality&f=false
It cites the Wickersham Report as setting off a trend towards closer scrutiny of police practices but says that things only really began to change in the 1960s, and particularly as a result of the Miranda decision, which almost completely eliminated police brutality during interrogations. (No matter what your friendly neighborhood right-wingers might say, Miranda is not some petty technical requirement.)
But somehow in the last few decades police brutality has cropped up again, only now as part of the arrest procedure and mainly directed against protests and civil disobedience. In that context, the purpose is not to extract confessions but to demonstrate that there is a heavy price to be paid for opposing the system -- which is to say that it has become an explicit tool of political repression.
I haven't found anything online so far about just how and when it became acceptable to torture protesters in the process of arresting them. My own recollection is that I first began to see discussions of pain compliance techniques in the late 80s or early 90s, about the same time as the "ticking time bomb" scenario began to show up on TV talk shows as a defense of torture.
But just where the responsibility for endorsing these abusive tactics currently lies -- whether among ordinary cops, their white-shirt superiors, the mayors and other city officials who are in the service of elite and corporate demands to promote gentrification and tourism, or high-level federal and state officials -- is a much trickier question.
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