Are airport employees security checked?
Many employees are not screened. This article, written in the aftermath of the LAX &quot;dry ice bomb&quot; incident, goes in detail about the levels of security employees face.

Ramp workers like 28-year-old Dicarlo Bennett, who was charged Thursday with two counts of possessing a destructive device in a public place, have access to just about anywhere in the airport. And though they must pass a 10-year background check before they’re hired, airport workers generally do not need to clear security when they arrive for their shifts.
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Melendez said it’s not possible to check every employee when they arrive each day, especially when many of those workers have access to potentially dangerous or disruptive items once they’re on the job. (Dry ice is regularly used by caterers to keep food cold.)
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Arif Alikhan, deputy executive director for homeland security at LAX, said the key is to have many layers of security. He said unannounced security checks are common, and inspectors might take a look at an employee’s lunch box or bags. Employees are asked to question who enters secure areas of the airport, and told to never hold doors open for each other.

After another incident in which employees smuggled firearms and drugs onto a Puerto-Rican bound flight, the Department of Homeland Security did a feasability study on 100% employee screening. Part of their findings were:

The concept of 100% airport employee screening is feasible. However, we do not believe it is realistic at this time. Regardless of the amount of funding, resources, and effort allocated toward this endeavor, vulnerabilities will persist. While the intelligence, screening, and law enforcement communities continue to make progress in addressing these vulnerabilities, 100% airport employee screening cannot entirely bridge existing gaps. As TSA changes its regulatory requirements, technology improves, and airports upgrade existing security measures, implementing this concept would be more realistic and should be revisited.

If you do a Google search on the subject, you will find numerous documents from large airports across the country detailing their procedures, most of which merely have to do with background checks and security badges.