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Mass murder is not a recent phenomenon in the U.S. As far back as July 26, 1764, a teacher and 10 students were shot dead by four Lenape Native Americans in Greencastle, Pennsylvania, in what generally is considered to be the earliest known mass school shooting.

Incidents of mass murder are well documented throughout the twentieth century. In the most comprehensive study of incidents committed during the twentieth century in the U.S., Grant Duwe examined 909 mass murders that took place between 1900 and 1999. He identified a distinctive and fascinating pattern of mass murder during those years (1).

The twentieth century was characterized by two mass murder peaks or waves. One of those waves appeared in the 1920s and 1930s and the second appeared in the mid-1960s and lasted until the mid-1990s. The period that separated the two peaks—that is, the decades of the 1940s and 1950s—represented a relatively tranquil phase in mass murder (2).

Mass murder incidents, particularly those committed in public places such as shopping malls, workplaces and schools have been high-profile news stories since the 1960s. This news trend dates back to the August 1, 1966, Texas Tower Shootings where student Charles Whitman climbed a 27-story tower on the University of Texas campus with three rifles, two pistols, and a sawed-off shotgun.

The 25-year-old architectural engineering major and ex-Marine—who had previously complained of searing headaches and —had already murdered his mother, Margaret, and his wife, Kathy, earlier that morning. He fired his first shots just before noon, aiming with deadly precision at pedestrians below. At 1:24 p.m., he was gunned down himself by the police. By the time Whitman was finished shooting, 14 people were dead and another 31 were wounded.

Several decades later, on April 20, 1999, high school seniors Eric Harris and Dylan Klebold shot up Columbine High School in Colorado. In addition to the shootings, the two troubled students’ complex and highly planned attack included 99 explosive devices such as a bomb to divert firefighters, propane tanks converted to bombs in the cafeteria, and bombs rigged in cars.

The perpetrators murdered a total of 12 students and one teacher in the slaughter. They wounded 21 others before shooting themselves in what has become known as the Columbine High School Massacre. Although their motives remain unclear to this day, the personal journals of the perpetrators document that they wished their actions to rival the Oklahoma City terrorist bombing, as well as other deadly attacks that occurred in the U.S. in the 1990s.

The Columbine High School Massacre has become the symbolic and definitive mass shooting of the late twentieth century in the popular culture. This is due in part to the critically acclaimed 2002 documentary Bowling for Columbine. This film examines and critiques an “American culture of and guns” according to its writer and director Michael Moore.

The film ignited a political debate over gun control laws and increased awareness of school shootings. Although mass shootings often receive heavy media exposure and create widespread concern, the Columbine High School Massacre became a lightning rod for public outrage in the late twentieth century due to the spectacular nature of the incident itself and the massive news media it received.

Mass Public Shootings Are on the Rise

Mass public shootings such as the movie theater massacre perpetrated by James Holmes, in Aurora, Colorado, in July 2012, and the October 1, 2017, Las Vegas mass shooting are currently on the rise in the U.S. An FBI study of “active shooter incidents” establishes an increasing frequency of mass public shootings (4).

The FBI report is based on 160 so-called active shooter cases which occurred between 2000 and 2013. The FBI defined an active shooter as one who is "actively engaged in killing or attempting to kill people in a public place,” regardless of the number of casualties that occur.

The FBI specifically excluded from its analysis family-related mass murders that occurred in private locations such as homes. The motive for murdering family members (familicide) in private is very different than the motive for murdering strangers in public.

During the first seven years included in the FBI study, an average of 6.4 incidents occurred annually. In the last seven years of the study, that average increased to 16.4 incidents annually. For example, in 2010, the U.S. had 26 mass public shootings, which is the highest number since 1999.

What can explain the recent rise in mass public shootings? I contend that there are powerful social forces today that promote mass murder. These include financial and healthcare fears, a declining belief in the American dream, a distrust of government, global terrorism and constant war since 2001.

These factors have led to alienation, and a feeling of powerlessness for many people. A few but increasing number of them are striking out in public acts of violence against complete strangers.

I examine the and habits of notorious serial killers, including the “Son of Sam” and “Bind, Torture, Kill” based on my personal correspondence with them, in my best-selling book Why We Love Serial Killers: The Curious Appeal of the World’s Most Savage Murderers.

Dr. Scott Bonn is a , professor, author and TV analyst. Follow him @DocBonn on Twitter and visit his website docbonn.com