Gangs in the country have about 1 million members, it says.
Although gang membership traditionally has been a trigger for violent crime, murder and other violent offenses have declined substantially in much of the country, USA Today reported.
Nationally, violent crime has sunk to its lowest levels since 1973, dropping from 19.3 incidents per 1,000 people in 2008 to 17.1 incidents in 2009, the National Criminal Victimization Survey said.
"With gangs usually comes a lot of violence; we're looking at this very closely," said John Moore, director of the National Gang Center, a branch of the Justice Department.
Authorities said the drop in murders and other violence is the result of the imprisonment of high-ranking gang members.
"We've taken out so many of the leaders -- stone-cold killers -- that it has fractured some of these organizations," said Chicago Police Cmdr. Leo Schmitz.
Chicago's murder rate is on pace to drop to its lowest levels since 1965. The 343 murders this year are eight fewer than this time last year, Chicago Police Superintendent Jody Weis said.
Los Angeles has about 45,000 gang members, up 2,000 since 2008, but murder and other violent crimes there are declining, the newspaper said.
This year's 232 Los Angeles murders are an 8 percent drop from the same time in 2009 and 24 percent under the 2008 total.
"We've been very fortunate so far," said Los Angeles police detective Jorge Luis Martinez. "We're going in the right direction."