In Timothy Egan's book "Breaking Blue," he talks about a depression era case where a Spokane police detective killed a local town marshall while committing a robbery. The detective's superiors weren't happy about the situation(the detective was later demoted), but they helped cover it up. It stayed covered up until the police officer who'd been ordered to dispose of the murder weapon confessed in the last months of his life (1989).

In the late 1980's, the sheriff of the neighboring county, Tony Bamonte, was writing his master's thesis about an old cold case in the county seat, Newport Wa.(Much of the information from his thesis was apparently published in Bamonte's book, see link below.) During the Great Depression, grassland had dried up, and due to the lack of feed, cattle were being sold in large numbers to the slaughter houses. As the numbers of dairy cows dropped, dairy prices increased, and butter in particular, became very expensive.

In 1935, the Newport Town Marshall, George Conniff came across a night robbery at the local creamery, and was shot and killed, with his murder remaining unsolved. As Bamonte's interest in the case attracted newspaper attention, a retired Spokane police detective, Daniel Mangan, contacted him, wanting to tell him about a gun he'd been ordered by his superiors to throw off a bridge into the Spokane River, which had apparently belonged to a detective who "got himself in trouble," and had been known as a rough character. This officer, Clyde Ralstin, had apparently been involved in other robberies, and was also selling bulk, presumably stolen, butter to local Spokane restaurants.

Bamonte was able to actually find the detective's pistol on the bottom of the Spokane River during a low-water period, and Egan(as a newspaper reporter) got what many viewed as essentially a confession from the 90+ year old detective. As this story began to unfold, the Spokane police chief at that time held a press conference railing against the country sheriff(Bamonte) for wasting taxpayers' money!

Egan also gives a good historical overview of the depression-era Spokane police, and reporting that most of the patrolmen hired the same year as Mangan were actually ex-loggers. The reason loggers were prefered was because they had a lot of muscle; Mangan told him that as patrolmen their orders were not to put people in jail, where the city would have to pay to feed them, but rather to administer punishment (beatings) on the spot, and make sure the offender left town. Egan also talks about the culture of petty bribery that Spokane police took from local businesses, as well as payments from bootleggers to look the other way.

While the Spokane Police department did cover this up, they also demoted the officer several months later(waiting for things to cool down?), and he quit the force a year later. What was an "acceptable" level of corruption? Not easy to answer, because by it's nature, it's something hidden.

Links: Unfortunately, Egan's book is the only good record of what happened. While he followed the story as a newspaper reporter, the archives of stories he wrote for the local papers aren't available online.

http://www.nytimes.com/1989/11/16/us/after-54-years-river-s-quirk-gives-up-a-clue-in-a-killing.html

(While the NY Times article is a decent overview, it compresses a lot of information, and botched the dates of the first police confession, it wasn't 1935, but rather 1955 or 1965, iirc.(I don't have the book at hand.) And a several of the details that article posted, particularly that confession only emerged after the story came out.)

Bamonties book: Sheriffs 1911-1989: A History of Murders in the Wilderness of Washington's Last County

Follow up question. If such an event has a historic basis what was the public reaction/backlash when it was discovered?

Man, the mods on this Subreddit sure are a bunch of censoring fascists. If only we didn't have such strict standards, the insights of this incredible thread would be available to one and all.

At the time of those screenshots, there were 21 comments in the thread. That there is 20 of them^1. That's what you're missing.

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^1 I actually can't spot which one I missed in the screenshot.