The FBI described the gang presence as a "growing threat," the Chicago Sun-Times reported.
"The military enlistment of gang members could ultimately lead to the worldwide expansion of U.S.-based gangs," the report said.
The Defense Department does not track gang membership in the military. But FBI investigators believe the reduction in enlistment standards -- due to recruitment pressure related to the Iraq War -- has brought more gang members in.
Recruiters are not trained to look for signs of gang membership, the report said. Others ignore criminal records of willing volunteers -- such as the recruiter who concealed the fact that a member of the Latin Kings was awaiting trial for a razor assault on a New York police officer.
The FBI said some gangs recruit veterans or serving members of the military because of their experience with explosives and firearms. Other gang members may join up to get access to weapons and equipment.