I remember I posted a figure on this, recently. Although not reliable (the number must be much higher), in 2012, over 400 people got killed by the police in the US.
I guess that's a very optimistic number.
About the current mass riots in the US:
Amanda M. Sakuma ✔ @iamsakuma
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Car driver offered to give a ride home to remaining protesters. Police stopped the car, guns drawn
10:06 PM - 18 Aug 2014
04:59 GMT: There are reports of a gunshot victim, who was wounded amid the altercation between police and protesters. It was not immediately clear whether the victim was an officer, one of the demonstrators, or a bystander.
04:45 GMT: Missouri police say live ammunition fired from protesting demonstrators, Vice News reports from the scene. The officers ordered the media to go to a designated safe area.
Ben Kesling @bkesling
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Police-fired tear gas returned by a protester in
#Ferguson
9:22 PM - 18 Aug 2014
04:21 GMT:
Amanda Wills ✔ @AmandaWills
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Amnesty International guy helping another man who was tear gassed pretty bad just now.
#Ferguson
9:19 PM - 18 Aug 2014 Ferguson, MO, United States
04:03 GMT: Police have started to forcefully disperse protesters in Ferguson with tear gas and stun grenades. Gunshots and the sound of breaking glass can be heard on live footage from the scene.
Danny Wicentowski @D_Towski
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Tear gas, flash bangs. So much for restraint.
#Ferguson
9:06 PM - 18 Aug 2014
Ryan Devereaux @rdevro
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Police: "All members of the media please separate yourself from the protesters immediately"
8:43 PM - 18 Aug 2014
02:51 GMT:
Maria Keena @kmoxmaria
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Hundreds of protestors face to face w/ police W Florissant & Ferguson
@michaelcalhoun #Ferguson
7:46 PM - 18 Aug 2014