Family, friends, teachers and survivors look back twenty years after Columbine, in an anniversary made tougher by a threat from a South Florida high schooler.

Since the devastating massacre at Columbine High School on April 20, 1999, the United States has seen more than 230 school shootings, not including ones at colleges or universities, according to data from the Washington Post.

How many school shootings were there in a given year? That depends on how they are defined.

Some methods of counting are more expansive than others. But by any count, 2018 saw a stark spike in school shootings.

Some organizations tracked shootings since Columbine or earlier, while others started counts following the tragedy at Sandy Hook Elementary School on Dec. 14, 2012, when a shooter killed 20 children and six school staff members.

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For example, Everytown for Gun Safety has tracked gunfire incidents at schools including colleges and universities, since 2013. Using press reports, the group counts "every time a firearm discharges a live round inside or into a school building" or onto a school campus or group. It does not include instances in which guns were brought into schools without being fired.

A school shooting tracker created by NBC News shows that there have been 38 shootings in schools, from kindergarten to college, since 2013. NBC News counts when an active shooter, with intent to harm, injures or kills at least one students or faculty member during the school or at a school event.

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The Center for Homeland Defense and Security at the Naval Postgraduate School takes another approach. Its database, which extends from 1970 to present, records “each and every instance in which a gun is brandished, fired, or a bullet hits school property for any reason, regardless of the number of victims (including zero), time, day of week, or reason (e.g. planned attack, accidental, domestic violence, gang-related).”