Active shooter incidents in the United States continue to rise in frequency, with new FBI statistics showing the highest average of incidents ever in a two-year period, reports ABC News.

The release Wednesday of the FBI's latest review of active shooter incidents came just three days after the worst mass shooting in American history, when a 29-year-old Florida man, identified by authorities as Omar Mateen, opened fire in an Orlando nightclub frequented by members of the gay community, killing 49 and wounding more than 50.

According to the FBI, 2014 and 2015 each saw 20 active shooter incidents. That's more than any two-year average in the past 16 years, and nearly six times as many as the period between 2000 and 2001, the starting point for the FBI's review.

The FBI says that the generally agreed upon definition of an active shooter situation is "an individual actively engaged in killing or attempting to kill people in a confined and populated area."

"Unlike a defined crime, such as a murder or mass killing, 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," the FBI said in a 2013 study of active shooter incidents.

Several high-profile incidents occurred last year alone -- from the terrorist attack in San Bernardino, CA, which killed 14, to the assault at a community college in Roseburg, OR, where seven were killed. Most of the 40 incidents in 2014 and 2015, however, didn't receive such national attention.