Police in New York have turned to Facebook as part of a crackdown on gang violence in the city.
Authorities said the social networking site is being used increasingly by a new generation of gang members to boast of their crimes and taunt their rivals.
The New York Police Department (NYPD) now plans to beef up its own online presence by doubling the size of its gang unit to 300 investigators.
Although much of the material they gather can be found on public Facebook profiles, the expanded team will also use more aggressive tactics.
Officers will adopt internet aliases, create phony profiles and "friend" suspects to gain access to non-public information.
NYPD Commissioner Raymond Kelly said: "By capitalising on the irresistible urge of these suspects to brag about their murderous exploits on Facebook, detectives used social media to draw a virtual map of their criminal activity over the last three years."
There are many examples of gang members using Facebook in an attempt to stir up trouble.
Police said one suspect posted: "Rockstarz up 3-0." It was believed to be a reference to the body count from a bloody turf war between the Brooklyn gang and another group, the Very Crispy Gangsters.
The Rockstarz also posted a photo of a Very Crispy member with the comment: "He is scared. Look at him."
Police and prosecutors insist the plans, which are due to be announced at a law enforcement convention in San Diego, follow strict legal protocols.
Donna Lieberman, executive director of the New York Civil Liberties Union, said the NYPD "has the right, indeed the obligation, to pursue effective avenues for investigating criminal gang activity, and that includes using Facebook and other social media".
But she said such methods must be closely monitored to ensure they do not become "a vehicle for entrapment or unauthorised surveillance".