(KSNF/KODE) — Getting through airport security is already stressful enough, but an additional screening method you may be subject to could add to that stress.

Any airline passenger with the letters “SSSS” printed on their boarding pass has been selected by airport security for extra screening. Used by the Transportation Security Administration, the letters SSSS stand for Secondary Security Screening Selection or Secondary Security Screening Selectee.

What Is ‘Secondary Security Screening Selection’ (SSSS)?

Iranian-born bioengineer researcher Nima Enayati holds up his boarding pass at Milan’s Malpensa International airport in Busto Arsizio, Italy, Sunday, Feb. 5, 2017. (AP Photo/Antonio Calanni)

According to the TSA, any passenger with the four-letter code on their boarding pass can be swabbed for the residue of illegal substances, such as drugs or explosives. Those passengers will also face additional questioning about travel plans and previous trips. The special security measures that come with an SSSS code can add an additional hour to the security check process.

The added check can be done either at the initial TSA screening area or the boarding gate, according to The Points Guy.

Carry-on bags are thoroughly examined — including the lining and exterior pockets — with everything removed and inspected. Any electronic devices may be required to be powered on for TSA inspection.

The added security measure was put in place following the 9/11 attacks, and its purpose is to make commercial aviation safer. However, the code isn’t exclusive to U.S. air travel. Those heading to or from airports in the U.S. are subject to having the four-letter code printed on their boarding pass.

Anyone selected for the additional screening will not be able to print their boarding pass in advance. That’s one indication that an SSSS code will be on the boarding pass when it’s printed at the terminal.

If you’re curious as to who is selected for SSSS, don’t waste your time searching the internet for a solid answer, especially from the TSA. There’s simply no published information about the selection — and for good reason. If someone is a threat to an airport or airline and knew how to avoid becoming a Secondary Security Screening Selectee, there’s a good chance that threat could become a reality.

Government Flub Reveals TSA’s Boarding Pass Code

According to an article published in “Wired,” the public first learned about the four-letter code in 2008, after government workers preparing the release of a 93-page TSA manual — which detailed airport screening procedures — posted a redacted version of the .pdf file online.

However, there was a big problem with the redaction process. The online magazine revealed the government flub in one of its feature stories. Instead of cutting out the redacted sections containing “sensitive security information,” the text was simply blacked out. This meant that anyone with a computer could easily reveal the redacted text by copying the blacked-out portions on the .pdf and pasting them in a document program (such as Microsoft Word).

A similar CNN article says despite the TSA’s quick efforts to remove the redacted version of the manual, it had already been distributed across many websites.