Donna Leinwand Leger

USA TODAY

The number of incidents in which a shooter opens fire on a crowd of people more than doubled over the past seven years compared with the previous seven, the FBI found in a study made public Wednesday.

The Federal Bureau of Investigation analyzed 160 "active shooter" incidents from 2000 through 2013 to look for common elements that might guide law enforcement officers in preventing the shootings or responding more effectively.

The study found an average of six incidents per year from 2000 through 2006. The number rose to 16 incidents annually in the past seven years.

The 160 incidents studied began with the Dec. 26, 2000, shooting at Edgewater Technology in Wakefield, Mass., when Michael McDermott, 42, armed with several weapons, shot seven of his co-workers to death. Police found him sitting in a conference room. Researchers also examined the shootings at Case Western Reserve University, ConAgra, Red Lake High School, Fort Hood, Virginia Tech, the U.S. Holocaust Museum and Sandy Hook Elementary School, among others.

The FBI study defined "active shooters" as a person or people "actively engaged in killing or attempting to kill people" in a "populated area." That is different from mass shooting incidents, which include any shooting in which more than three people are killed. It also excluded domestic violence and drug and gang-related violence. Of the 160 incidents studied, 64 fit the federal definition of mass killing.

Criminologist James Alan Fox disputes the FBI conclusion. Active shooting and mass shooting events are rare, and the data are too limited to conclude that active shooter incidents are increasing, says Fox, a professor of criminology at Northeastern University in Boston.

"Unlike mass shooting data, which come from routinely collected police reports, there is no official data source for active shooter events," Fox says. "It's not clear whether the increase in active shooter events is completely related to the actual case count or to the availability and accessibility of news reports to identify such events."

USA TODAY catalogued all mass killings -- more than three killed by whatever means -- from 2006 -13 and found FBI homicide data just 57% accurate in identifying them. USA TODAY's broader review showed the number is not increasing. From 2006 to date, 258 have occurred, about one every two weeks. More than half occur among members of a households and intimate partners, and 25% do not include a gun.

More than two-thirds of the incidents analyzed by the FBI happened at businesses or schools. In many cases, they happened so quickly that the shooting was over before police arrived. Of 64 incidents analyzed, 44 were over in five minutes or less. Of those, 23 ended in two minutes or less.

In 28% of the incidents, police exchanged fire with the shooter, the study found. In nearly half of those incidents, police were killed or wounded. The shooter committed suicide at the scene before police arrived in 23% of the cases. Unarmed civilians successfully restrained the shooter in 13% of the cases.

The study found few common themes among the shooters. In about 10% of the incidents, male shooters targeted current and former wives and girlfriends but also shot bystanders. In about 9% of the incidents, shooters targeted family members.

In all but two incidents, the shooter acted alone. All but six of the shooters were male.The analysis found no age pattern but noted that in the vast majority of school shootings, the shooter was a student at the school.

The study did not address access to guns.

Almost all of the shooters had a "real or perceived, deeply held personal grievance," said Andre Simons, unit chief for the FBI's Behavioral Analysis Unit 2.

Many of the shooters took inspiration from attacks by other shooters, such as Columbine and Virginia Tech, Simons said.

"The copycat phenomenon is real," he said.

The FBI plans to study the data to identify behavior that might indicate that a person is heading toward committing a violent act and then educate people to see the warning signs, Simons said.

FBI Assistant Director James​ Yacone said he hoped the FBI could use the data to better prepare local police departments to respond to active shootings, determine what type of equipment could better protect them and learn how to identify and stop people who might resort to such violence.

"The motivations vary widely, but if there are themes, if there are pre-attack indicators, we want to identify them," Yacone said.