FBI: Mass shooting incidents occurring more frequently

WASHINGTON (CNN) -- Mass shootings have been occurring more frequently in recent years, an FBI study shows, with nearly one incident a month from 2000 to 2013.

The study, which was released Wednesday, reported 160 incidents during that time, with 486 people killed and 557 wounded. Those figures don't include the deaths or injuries of the shooters.

The bureau called such shootings "active shooter incidents" -- which the report described as "individuals actively engaged in killing or attempting to kill people in populated areas (excluding shootings related to gang or drug violence)."

Underscoring the difficulty authorities have to prevent such incidents or reduce their lethality, the study found that 60% of the shootings ended before law enforcement officials arrived. In cases where investigators could determine the length of time of the incident, most of the shootings lasted under five minutes and many under two minutes.

In 40% of the cases studied, the shooters killed themselves.

Most of the shootings were carried out by males. Only six of the "active shooters" in incidents during that time period were female.

Seventy percent of the incidents "occurred in either a commerce/business or educational environment," the FBI said.

Frequency increasing

And if you think such shootings seem to be happening more frequently in the past few years, you're right.

"In the first half of the years studied, the average annual number of incidents was 6.4, but that average rose in the second half of the study to 16.4, an average of more than one incident per month," the study said.

"Recognizing the increased active shooter threat and the swiftness with which active shooter incidents unfold, these study results support the importance of training and exercises -- not only for law enforcement but also for citizens."

The FBI said the goal of the study is to "provide federal, state, and local law enforcement with accurate data so they can better understand how to prevent, prepare for, respond to, and recover from these incidents."

Active shooter incidents differ from "a defined crime, such as a murder or mass killing," the study says, because "the active aspect inherently implies that both law enforcement personnel and citizens have the potential to affect the outcome of the event based upon their responses."

Case studies

Among the incidents studied by the FBI for the report was the Sandy Hook Elementary School shooting in Newtown, Connecticut, on December 14, 2012. Gunman Adam Lanza, 20, killed 20 children, ages 6 and 7, and six adults at the school before killing himself. Police later found the body of Lanza's mother at her home.

"In at least nine incidents, the shooter first shot and killed a family member(s) in a residence before moving to a more public location to continue shooting," the FBI report said.

Another case studied for the report was the mass shooting at an Aurora, Colorado, movie theater in 2012. Twelve people were killed and 58 wounded. Accused gunman James Holmes pleaded not guilty by reason of insanity to the killings; his trial begins next month.

In 2007, Virginia Tech student Seung-Hui Cho shot and killed 32 people and wounded dozens more in a classroom building and a dorm on campus before killing himself.

Maj. Nidal Malik Hasan killed 13 people and wounded 32 at Fort Hood, Texas, during a shooting rampage November 5, 2009. He was convicted and sentenced to death.




CNN's Alanne Orjoux contributed to this report.