(CNN) - The Transportation Security Administration says the longer the government shutdown goes on, the greater the staff shortages the agency will experience.

TSA workers and air traffic controllers are considered essential employees, so they must keep working, even though they are not getting paid.

The employees will get back pay after the shutdown ends.

The TSA says since agents aren’t getting paid but still have bills coming in, as time goes on, it will get harder and harder for them to show up for work.

The TSA also acknowledged a staffing shortage caused long lines for security screening last weekend in Atlanta.

The partial shutdown started Oct. 1 when Congress could not agree on a funding bill.

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