c. 2015 USA Today

In the early hours Thursday morning, residents shaken to the core in Charleston, South Carolina, held hands and formed a circle of prayer down the street from what was another mass shooting in the U.S.

It has happened before in a kindergarten class, a college campus, a movie theater, a McDonald’s. The words resonate as places forever linked with the unspeakable: Sandy Hook, Columbine, Virginia Tech, Luby’s.

This time it was the Emanuel African Methodist Episcopal Church, the oldest AME church in the South, where people had gathered for a prayer meeting Wednesday night.

“I do believe this was a hate crime,” Police Chief Gregory Mullen said after confirming that nine people were dead. “This is a tragedy that no community should have to experience. . . . It is senseless. It is unfathomable that someone would walk into a church when people are having a prayer meeting and take their lives.”

Said Charleston’s mayor, Joe Riley: “It is the most intolerable and unbelievable act possible. The only reason someone could walk into church and shoot people praying is out of hate.”

Since 2006, there have been more than 200 mass killings in the U.S. The FBI defines a mass killing as an incident with four or more victims.

A USA Today special report, Behind the Bloodshed, documented how mass killings occur about every two weeks. Public massacres such as the 2012 shooting at Sandy Hook Elementary School in Newtown, Connecticut, account for one in six mass killings.

The report also documented how the majority of mass killings are family-related. Seventy-seven percent of mass killings involve a gun, and nearly three out of four guns involved were handguns, USA Today found.

One of the most stunning revelations in the report: A mass killing often involves a failed safety net, such as protective orders that didn’t work or gaps in the mental health system.