Number of active shooter incidents surges in recent years, FBI study finds

New data just released by the FBI show that active shooter incidents in the United States continue to rise, with last year and the year before having the highest average of incidents ever in a two-year period.

WATCH: What to do in an active shooter situation

A previous FBI study of such shootings found that between 2000 and 2001, an average of 3.5 incidents occurred annually. Now - in years 2014 and 2015 - that number has multiplied nearly six times, to an average of 20 active shooter incidents each year (in a two-year period). See here how the average incidents in two-year periods have grown over the past 16 years:

AVERAGE NUMBER OF ACTIVE SHOOTER INCIDENTS IN TWO-YEAR PERIOD

2000-2001: 3.5 incidents
2002-2003: 7.5
2004-2005: 6.5
2006-2007: 12
2008-2009: 13.5
2010-2011: 18
2012-2013: 19
2014-2015: 20

Here are those same numbers in graph format:

The FBI defines an active shooter incident as 'an individual actively engaged in killing or attempting to kill people in a confined and populated area.' Implicit in this definition is that the subject's criminal actions involve the use of firearms.

Some other new stats from FBI on incidents and shooters from 2014 and 2015:

- Over the two years, 92 killed, and 139 wounded (not including the shooters) in a total of 40 incidents.

- Three female and 39 male shooters conducted the 40 incidents. Husband-and-wife teams carried out 2 different incidents. One shooter was a preteen, 5 were in their teens, 16 were in their 20s, 5 were in their 30s, 8 were in their 40s, 5 were in their 50s, 1 was in his 60s, and 1 was in his 70s.

- Sixteen (38.1%) of the 42 shooters committed suicide: 7 at the scene before law enforcement arrived, 7 at the scene after law enforcement arrived, and 2 at another location.

- Fourteen (33.3%) of the 42 shooters were killed by law enforcement, including 1 killed by an off-duty officer. Twelve of them were killed at the scene.

- Twelve (28.6%) of the 42 shooters were apprehended, 7 by law enforcement alone and 5 through citizen involvement.

ABC News contributed to this report.