The mass murder last week at the Pittsburgh Synagogue has something in common with the deadliest massacres: the AR-15 semi-automatic rifle. Variations of the AR-15 were used to kill at a Texas church, a Las Vegas concert, Marjorie Stoneman Douglas High School in Florida and Sandy Hook Elementary School. The AR-15 style rifle is the most popular rifle in America. There are well over 11 million, and they are rarely used in crime. Handguns kill far more people, but the AR-15 is the choice of our worst mass murderers. AR-15 ammunition travels up to three times the speed of sound. And tonight we're going to slow that down - so you can see why the AR-15's high-velocity ammo is the fear of every American emergency room

Mass shootings were once so shocking they were impossible to forget. Now they've become so frequent it's hard to remember them all. Last Saturday, in the Pittsburgh Synagogue, 11 were killed, six wounded.

FBI Special Agent Robert Jones: This is the most horrific scene I've seen in 22 years with the Federal Bureau of Investigations. Members of the Tree of Life Synagogue conducting a peaceful service in their place of worship were brutally murdered by a gunman targeting them simply because of their faith.

Just 11 months before, it was a church in Sutherland Springs, Texas. Assistant fire chief Rusty Duncan was among the first to arrive.

Rusty Duncan: 90 percent of the people in there were unrecognizable. You know the blood everywhere, I mean it just covered them from head to toe. They were shot in so many different places that you just couldn't make out who they were.

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