Looking back, background checks did not stop three mass shootings that claimed more than 40 lives since 2011.
In the massacre in Newtown, Connecticut, where 26 people, mostly children, were gunned down at Sandy Hook Elementary School, Adam Lanza used two semi-automatic handguns and a semi-automatic rifle. He didn't get a background check for those weapons. They were legally purchased and registered to his mother, Nancy Lanza, who was his first victim.
Last July, James Holmes walked into a crowded theater in Aurora, Colorado, and began shooting. His AR-15, two 9 mm Glocks, .40 caliber pistol and 12-guage shotgun were all purchased legally, after his name was submitted to the National Instant Criminal Background Check System (NICS). Twelve people were killed.
The same is true with Jared Loughner, who shot then-Rep. Gabrielle Giffords and 19 other people, killing six, in Tucson in January 2011. His application was never flagged when run through NICS -- the database from which potential firearms buyers are determined legally eligible to purchase a gun.
http://www.cnn.com/2013/04/10/politics/background-checks-mass-shootings/index.html?iid=article_sidebar