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.