It's been 3 years since one of the deadliest massacres in US history - there have been over 1,000 mass shootings since

Three years ago Monday, a shooter opened fire in a Connecticut school, killing 20 children and six others in the second-deadliest recorded mass shooting in US history.

Three years later, the number of communities that have had to endure mass-shooting incidents remains staggering.

The Mass Shooting Tracker is a crowdsourced Reddit database sourced largely from media reports listing every incident in the US in which four people have been wounded or killed by gunfire. (This differs slightly from other definitions - like that of the FBI, which defines a mass shooting as an incident in which four or more people are killed.) 

According to the tracker, there have been at least 1,052 such mass-shooting incidents between 2013 and the time of this writing:

The Sandy Hook Elementary School massacre served as an inflection point for a country that was left shocked by the brutality of the incident. But its aftermath eventually devolved into a charged debate over stricter controls on guns. And three years after the massacre, the US has taken strikingly few steps to prevent more from occurring. 

In another stunning data point, an NBC analysis released Monday found that 554 children under the age of 12 have died as a result of gunshots since the Sandy Hook massacre. It works out to a child under 12 dying by gunshot about once every other day - and NBC noted its numbers may be low because of the unavailability of certain data.

"We grieve today, as we have every single day since that soul crushing December morning," said Sen. Chris Murphy (D-Connecticut), who was a US representative in the Newtown, Connecticut, district at the time of the 2012 massacre.

"We remember the little ones, and marvel at what they were, and what they could have become. We recall with awe those brave educators, who tried to protect them, and perished in the effort," he added. "And we never stop searching for answers. Why did it happen? Did it have to happen? What can we do, now, to make sure it never happens again?"