It is difficult, and perilous, to attempt to prosecute individuals or groups solely on the basis of their association. The right to peaceful assembly and the freedom of speech protect types of conduct that can be abused, but which are protected by the United States Constitution.
Gang activity, especially involving the larger gangs, often cross jurisdictional boundaries between cities, states, and the federal government. Each level of administration has its own way of combating gang activities that threaten the public. Violent activity, not surprisingly, is the easiest to prosecute because violent acts are in and of themselves illegal except under extenuating circumstances, for example, self-defense.
Cities and states attempt to combat or control gang activity through laws and ordinances that prohibit types of activities frequently associated with gangs. Laws banning graffiti, for example, are used to contain such activity, as street gangs routinely mark their territory with graffiti spray-painted on walls -- usually publicly-owned walls, but also those of small businesses in the affected neighborhoods.
Street gangs are usually easy to identify because individual gangs adopt certain modes of dress. In Los Angeles, for example, the well-known conflict between the red-clad Bloods and the blue-attired Crips makes both easy to identify. Tattoos also are identifying marks used by gangs.
Gangs that use identifying apparel or easily-visible tattoos routinely use violence, but their appearances are threatening enough that they can intimidate merely by showing-up. This has the disadvantage of making it much easier for law enforcement agencies to identify them. When apparel routinely associated with gang activity is spotted, it makes it legally much easier for police officers to question such individuals without having to witness a crime.
The federal government similarly has statutes for dealing with gangs. Because some of the larger street gangs operate nationally -- in effect, gang activities cross state lines -- the federal government has jurisdiction to investigate and prosecute. Once again, violent acts are easy to prosecute because of their unambiguous nature, although prosecutions often hinge on the testimony of witnesses fearful of being targeted by the gangs in retaliation for that testimony.
One of the most important tools the federal govenment possesses for combating gang activities is the series of federal statutes known as the Racketeer Influenced and Corrupt Organizations Act (the RICO Act). Originally drafted and passed into law as a means of prosecuting organized criminal activity like that conducted by the Mafia, RICO's use has since been expanded to cover additional areas of criminal activity where a formal or informal organization exists. RICO allows law enforcement to target entire organizations, not just individuals. Through its application, the sum of criminal activity becomes greater than its parts.
These, then, are the ways city and state governments and the federal government combat gang activity.