Reasons

Anyone who will work with sensitive information or material that could compromise national security must be cleared before doing so. Military personnel and government workers normally need a security clearance.

Investigations

The type of investigation conducted on an individual is dependent on the level of security clearance needed. That, in turn, depends on the type of information or material the person in question will be handling.

Confidential

A confidential security clearance is the lowest level. Normally, a National Agency Check will be run to see if the individual has a criminal record. A security breach at this level would cause the least harm to national security.

Secret

A secret clearance is the next level. Often, a National Agency Check will be conducted, but under certain circumstances, a background investigation must be performed. More serious harm could be sustained with a breach at the secret level.

Top Secret

A top secret clearance is the highest level normally required. A minimum of a background investigation must be performed because very serious harm to our national security could result from damage at this level.

Source:

TAOnline.com

U.S. Department of State

Army.com

Resource:

Federal Bureau of Investigation