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Violent action won't help. You will be arrested and likely beaten as well. If you physically attack the cop it might actually make it worse for the guy you are trying to protect.
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Here's what I would do, keeping in mind that this is my personal opinion and not legal or tactical advice. I just want to give you some options so you don't have to stand there and watch someone get killed.
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As you approach, walk slowly with your hands in plain view. Don't move in too close. You don't want the officers to see you as a threat. Move close enough to talk to the officer, but not close enough to touch.
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Probably the best thing to do is to approach another officer on scene who has less ego involvement rather than the kneeling dude.
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Say something like: "Hey officer, I just want to let you know that the guy on the ground appears to be suffering from a medical condition. I don't know if the officer controlling him knows he's kneeling on the dude's neck. People are videotaping and it doesn't look good. I just don't want you guys to get in trouble."
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If someone approached me at a similar scene in that manner, I would most certainly go check things out and ensure that the prisoner is OK.
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If there is no one else, I'd approach the officer and focus on the medical issues. "Officer, let me help you. I've had advanced medical training and that guy doesn't look so good. Let's move him on to his side and away from the car so that he can breathe better and I'll check him out for you."
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Another way that might work is "Officer, are you OK? I'm a martial arts instructor. Can I help you hold him down so that you don't have to kneel on his neck? Just tell me what you want me to do and I'll do it."
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I think those strategies might have a high likelihood of success, but ultimately if the officers refuse your aid, there isn't much else you can do.
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As a last resort, I'd call 911 and tell the dispatchers that the officers on scene are in danger of killing their prisoner and request that both a supervisor and an ambulance respond to the scene. I'd say that might have a 50/50 chance of working depending on who the boss is and how busy they are.
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It's a bad situation. I think these strategies might be your best bet. If you can think of something better, please share your tactics in the comments below.