S.W.A.T.
Special Weapons and Tactics. A special group of handpicked expert policemen and/or ex-soldiers, these people are only called upon when the situation gets "tactical." Specializing in complex operations, such as hostage rescue, riot control, and urban assaults, they are armed with watered down military weapons and specialized equipment, including, but not limited to, C2 door breaching charges, Kevlar Outfits, GSG-9 boots, Gas Masks, Tear Gas, Tazers, Pepper Spray, LTL (Less Than Lethal) weaponry, among other things. They are also trained in various close quarters self defense techniques.

The S.W.A.T. are encouraged not to kill, if possible, and are only authorized to only return fire when either 
a) Under fire 
b) An innocent/teammate is in danger

However, there have been cases where they have overexcercised there right to kill. 

To put it simply, just don't fuck with them.
The S.W.A.T. have saved countless lives and arrested hundreds of criminals to date.