Millions of Americans may soon need a second form of ID to get through airport security. Turns out, driver's licenses from four states and one U.S. territory aren't compliant with federal security standards.

The REAL ID Act -- passed by Congress 10 years ago -- set minimum security standards for state-issued driver's licenses and photo IDs. But enforcement of the new regulations wasn't announced until 2013 -- and part of the final phase of enforcement includes how people access commercial aircrafts.

If state IDs don't comply with the new standards, federal agencies can't accept them as a person's sole proof of identification. And according to the DHS, full enforcement is set to begin "No sooner than 2016."

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"No sooner" makes the realistic timeline a little confusing. So enforcement could begin next year, or not. And while the exact timeline of enforcement is still unclear, the DHS does say this:

“DHS will ensure the public has ample advance notice before identification requirements for boarding aircraft change. That notice will include information on the process for individuals with a non-compliant driver’s license or identification card to be able to travel by aircraft.”

Driver's licenses from 4 states could soon be useless for getting through airport security

Only about half of U.S. states and territories impacted by the new rules are currently REAL ID compliant -- meaning residents with IDs from those states won't be affected. And most states that aren't yet compliant with the new standards have extensions in place that will allow their residents to continue using their current non-compliant IDs.

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But according to USA Today, there are four states -- Louisiana, Minnesota, New Hampshire, and New York -- that are not yet in compliance with the new rules, and residents from these states may be required to have a second form of ID in order to get past TSA.

So while it's still unclear when the new standards will actually be enforced, people from these four non-compliant states should plan to carry a second form of ID -- such as a passport -- if they plan to fly anywhere beginning in 2016.