Mass shootings have occurred in the United States with increasing frequency over the past 14 years, with 486 people killed in 160 incidents, according to a Federal Bureau of Investigation analysis released on Wednesday.

There were an average of 16.4 active-shooter incidents from 2007 through 2013, more than double the 6.4 average from 2000 to 2006, the report found.

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Active shootings, which the report defined as incidents not directly linked to gang violence or drugs and where police and citizens had the time to influence the outcome, occurred most frequently in places of business, which saw 46 percent of the incidents, followed by schools, where 24 percent occurred.

The report was released a day after a man who had recently been fired from his job at a United Parcel Service Inc distribution center in Birmingham, Alabama, shot dead two supervisors at the site before turning the gun on himself.

That ending, the gunman taking his own life, was a common one in the events the FBI analyzed, with 40 percent of the shooters turning their guns on themselves. Police shot and killed the attacker in 13 percent of the cases analyzed.

The deadliest incidents were the 2007 shooting at Virginia Tech University, where 32 people were killed, and the 2012 massacre at Sandy Hook Elementary School, where 27 died excluding the gunman.

FBI officials noted that many shooters had studied past incidents, particularly prominent ones like Virginia Tech, Sandy Hook or the 1999 attack at Columbine High School in Colorado, where two gunmen killed 12 students and a teacher.

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That incident was not included in the 160 counted by the FBI.

“Many offenders look to past offenses, particularly notable ones such as Columbine and Virginia Tech and Sandy Hook, and they study and they research those attacks,” Andre Simons, an FBI supervisory special agent, told a briefing in Washington.

“They look back to these past offenses and they oftentimes find inspiration and they oftentimes seek to emulate or copycat those particular offenses. So the copycat phenomenon is real,” he said.

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In 13 percent of the cases, unarmed bystanders or school staffers confronted the gunman, ending the shooting by subduing the attacker until police could arrive.

The FBI said that suggests both police and civilians would benefit from training on how to handle shooting incidents.

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“It is important, too, that training and exercises include not only an understanding of the threats faced but also the risks and options available in active shooter incidents,” the report found.

Richard Parker, a lecturer at Harvard University’s John F. Kennedy School of Government, noted the numbers of mass shootings are minimal compared with the roughly 14,000 murders and 1.15 million violent crimes reported in the United States per year.

“They get more attention than they statistically represent because they are so rare,” Parker said. “You have to ask where are the bulk of murders taking place, and this doesn’t even qualify as a significant minority. This is a tiny, tiny fraction.”

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(Reporting by Scott Malone in Boston; Additional reporting by Julia Edwards in Washington; Editing by Eric Beech)