Depending on how you define "mass shooting" the U.S. has had 121 from Aug. 1st 1966 til now, the total number of deaths from all these events is 794.

Here's an article with nice graphics to go along - 50 Years of Mass Shootings in America(WashPo)

Since January 1st until August 9th, 2015 the numbers of deaths at the hand of a police officer were already up to 1,083.

Here's another fancy page for that - Michael Brown Anniversary(Vice)

Catching up with the present. Since Jan. 1st of 2015 till Dec. 3rd, that toll is now 1,096.

All this contained in 1,087 separate events. I'm hoping that number is accurate, I counted each cell in the spread sheet below.

-Killed By Police

It only took 1 year for police to match, and then surpass the number of deaths caused by active shooters within the last 50.