Washington (CNN) -- Criminal gang membership increased as much as 40 percent in the United States during the past three years, according to an FBI report released Friday.
An estimated 1.4 million people are active in more than 33,000 street, prison and outlaw motorcycle gangs across the country, the report states. That compares to about 1 million gang members when the investigative agency last compiled such figures in January 2009.
FBI officials, however, were quick to emphasize that the apparent 40 percent increase in membership may reflect more accurate reporting of numbers, in addition to any sudden swell in gangs' ranks. They said some of the spike could be due to much better reporting by law enforcement officers, an increase in the number of agencies reporting and other administrative factors.
"We know gang membership has increased. How much, we really don't know," a senior FBI official told reporters at the agency's Washington headquarters.
The 100-page 2011 "National Gang Threat Assessment" claimed criminal gangs pose a growing threat in communities throughout the United States.
"The most notable trends for 2011 have been the overall increase in gang membership and the expansion of criminal street gangs' control of street-level drug sales and collaboration with rival gangs and other criminal organizations," according to the report.
While there was no data related to how the economy might factor into growth in gangs, more aggressive recruiting and cultural and ethnic factors may have contributed to the increase in gang membership.
Although overall crime in the United States has continued to decline over the past three years, the relative amount of crime inflicted by gang members appears to have increased. The new FBI report claims that gangs are responsible for 48% of violent crime, on average, in most jurisdictions.
Neighborhood-based gangs have proven that they can be as violent as the notorious Central American gang MS-13, which continues to grow. U.S.-based gangs are establishing stronger working relationships with Central American and Mexican gangs to facilitate not only drug smuggling, but also weapons trafficking and immigrant smuggling, the report claims.
Although anti-gang officials said at Friday's press conference that they are concerned about gang members who enter the military, they don't note any evident gang activity among active U.S. military personnel. However, they fear those individuals who receive weapons and tactical training might use both to commit violence crimes when they re-enter society.
Officials said that some gang members have radicalized while in prison, expressing worries that those individuals could be even more dangerous when they're released.
Although some view criminal gangs as susceptible to payments from would-be terrorists, the anti-gang officials said today's gang leaders are often sophisticated and understand the risk of getting caught up in any terrorist organizations or plots. To date, officials have not noticed any clear connections between criminal gangs and terrorist groups.
Gangs can be found in every U.S. state and the District of Columbia. The largest number of gang members is in southern California, southern Arizona and the Chicago area. The vast majority of these people belong to street gangs, predominantly in the West.