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Getting a top secret clearance like the one NSA leaker Edward Snowden had is supposed to require four traits: loyalty, character, trustworthiness and reliability.

But with an estimated 1.5 million Americans holding the Top Secret clearances, it is a virtual certainty that Snowden is not the only one with a pass that the government can't trust.

The clearances are bestowed upon government workers as well as employees of private contractors like Snowden, who provided reporters with highly classified documents, including PowerPoint Presentations that revealed the existence of the National Security Agency’s PRISM operation.

In the case of Snowden, who is believed to never have been in trouble with the law, the most damning thing to emerge about his past was a description posted on the now-defunct website Ryuhana Press. A then 18-year-old Snowden described himself as liking guns, food, girls, his “girlish figure that attracts girls…That's the best biography you'll get out of me, coppers!”

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Whether investigators dismissed the digital trail as the innocent musings of a teenager or missed it altogether is unknown. What is clear is that Uncle Sam did a lot of digging into his past, and came away confident he could keep secrets.

Recipients of Top Secret clearance undergo an extensive investigation, which includes a review of everywhere they have lived, attended school and worked, according to the General Accounting Office.

Investigators also interview four people who know the subject socially, talk to former spouses and check financial records. In 2011, the U.S. spent $1 billion to conduct background investigations for a variety of classifications. But any background check is only as good as the investigator who conducts it, according to FBI investigator William Daly.

“It is always up to the investigator’s due diligence to make sure they look at all aspects of the individual’s background and personality,” Daly told FoxNews.com.

For a government employee or private contractor like Snowden, there are three levels of security clearances: “Top Secret,” “Secret,” and “Confidential.” The U.S. government employs nearly five million people whose job requires some type of security clearance, and as of October 2012, 1.5 million people had “Top Secret”  clearance. After getting an offer of employment, each applicant must fill out a security questionnaire, such as the Questionnaire for National Security Positions, “thoroughly, honestly, and with candor,” according to the U.S. Department of State website.

The 127-page questionnaire asks for general information, such as name, whether the person is a U.S. citizen, school history and employment history.  The 29-part questionnaire also asks for specific information, including all the residences the applicant has lived at during the past ten years, names of relatives, foreign contacts and whether they have ever knowingly engaged in activities that could overthrow the U.S. government.

An in-depth investigation is then done, but how in-depth depends on the type of access a person needs to perform his or her official duties.  For access to “Confidential” or “Secret” information, checks on the applicant’s criminal and investigative file with the FBI and other national agencies are required, as well as checks on the applicant’s credit history.

For access to “Top Secret” information or “Sensitive Compartmented Information,” a more stringent and comprehensive check on the applicant’s history is performed.  The Single Scope Background Investigation includes: a subject interview, reviews of the applicant’s employment, education, residence, references, local law enforcement, court records, and national agency checks.