Per the book "America's Film Legacy: The Authoritative Guide to the Landmark Movies in the National Film Registry", the running time for this is 20 minutes.

Whom does the police protect, the people, the government or big companies? In the Memorial Day massacre of 1937, the Chicago Police Department shot and killed ten unarmed demonstrators in their back and their side as they where fleeing (and shot and injured 30 other. Nine people were permanently disabled and another 28 had serious head injuries from police clubbing). The police got the tear gas and the iron clubs they used from the steel factory! No police were ever prosecuted. The police said that their actions where justifiable because some of the people where on drugs! The Newsreel Footage shown in this short documentary was suppressed to hide what really happened:

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This is the closest I've been able to find - about two minutes:
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