Airport enforcement authority
While some countries may have uniform protection at all of their airports, in other countries like the U.S., the protection is controlled at the state or local level. The primary personnel will vary and can include:
A police force hired and dedicated to the airport
A branch (substation) of the local police department stationed at the airport
Members of the local police department assigned to the airport as their normal patrol area
Members of a country's military
Members of a country's airport protection service
Police dog services for explosive detection, drug detection and other purposes
An Armed police sergant at Hong Kong International Airport.
When additional personnel are required, then several of the groups listed above can be used and as required supplemented by other resources that include:
Officers from the normal agency, but in larger numbers using personnel not normally assigned to the airport
Security guards
Paramilitary forces
Reserve military forces
The Aviation and Transportation Security Act generally required that by November 19, 2002 all passenger screening must be conducted by Federal employees. As a result, passenger and baggage screening is now provided by the Transportation Security Administration (TSA), part of the Department of Homeland Security. Provisions to improve the technology for detecting explosives were included in the Terrorism Prevention Act of 2004.
http://en.wikipedia.org/wiki/Airport_security