(Cleveland) - With the stabbings of students at a school near Pittsburgh, we are reminded that April can be a tragic month in America. It marks the anniversaries of some of the most gruesome attacks in U.S. history.

April 19, 1993: the FBI's siege of the Waco compound leaves 76 dead.

April 19, 1995: the bombing of a federal building in Oklahoma City kills 168 people.

April 20, 1999: A mass shooting at Columbine High School leaves 15 dead.

April 16, 2007: The Virginia Tech massacre kills 33.

April 15, 2013: - Two separate bombs explode at the Boston Marathon, killing three people and wounding about 170 others.

Is there something about April and the start of spring that drives some people crazy?

Nationally recognized crime expert Dr. Daniel Flannery of Case Western Reserve University says there are several factors at play, especially in school violence.

Flannery is director of the Begun Center for Violence Prevention Research and Education at CWRU's Mandel School of Applied Social Sciences.

As far as school attacks are concerned, Flannery says April can be a stressful time for students facing academic achievement testing. It's also a transition time, with students and teachers coming and going. Kids without strong connections can feel lost and confused. Couple that with social media and Flannery says there is an increased potential for trouble.

Flannery also says there is in a trend in the media, to do stories about the anniversaries of tragic events, often highlighting the acts of the perpetrators, with victims and first responders receiving less attention. So anyone who is thinking about acting out and bringing attention to themselves are reminded of exactly how to do it.

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(Photo by Ken Robinson/WTAM)

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