The Days of Getting Free TSA PreCheck Are Almost Over

— -- Many U.S. travelers have experienced the luck of arriving at the airport and glancing at their boarding passes to see they have been granted access to the TSA PreCheck security lanes without having enrolled in the program.

Today the Transportation Security Administration said those days are numbered.

Starting this month, the agency's program will narrow its focus to those who have signed on.

"This is part of the natural progression of the TSA PreCheck program," TSA spokesman Michael England told ABC News. "In the future, we intend to only have enrolled or pre-vetted passengers or those screened by K-9s in the expedited screening lanes."

The program, which allows vetted passengers access to expedited screening lanes, has been steadily growing since the TSA opened the program to the general public in 2013.

There are more than 12 million enrollees in U.S. trusted-traveler programs such as PreCheck and Global Entry.

The programs allow the TSA to "focus more attention on higher risk passengers and those we know less about," said England.

He said the agency does not expect lines to be affected by today's announcement.