New York City Police Department

The New York City Police Department (NYPD), started in 1844, is the largest police force in the United States. It is the police force that serves the five boroughs of New York City. The NYPD was one of the first "modern" style police departments in the United States with the Boston Police Department. The NYPD uses a color of the day to allow uniformed officers to recognize undercover officers to prevent accidental shootings. 
Street Crimes Unit.
The New York Police Department's Street Crime Unit (motto: "We Own The Night") was a 300+ member plain clothes unit for reducing crime that became well known after the 1999 killing of Amadou Diallo. The four officers who did the killing were all members of the unit. The unit was ended in 2002 because of the Diallo shooting. The unit's last leader was Inspector Bruce H. Smolka, who was later made Assistant Chief. The Street Crime Unit is now replaced by local precinct Anti-Crime Units.
All of the officers were found not guilty of the criminal charges in a long trial that took place in Albany after a successful request to change the location of the trial from the Bronx, where the shooting happened.
The Street Crime Unit was started in 1971 and had a long history of success in catching armed criminals on NYC streets. The establishment of the Street Crime Unit led to the development of the color of the day undercover officer recognition system.