The nation’s airports continue to operate, even as parts of the government remain shut down: air-traffic-control workers and airport security officers remain on the job. But as the shutdown moves into its third week, some fear it’s only a matter of time before the nation’s air-traffic system begins to feel the impact.

Transportation Security Administration officials acknowledge that growing numbers of security screeners are not showing up for work but say the call-outs aren’t significant enough to have an impact on airport operations.

According to TSA officials, roughly 51,000 employees are involved in the airport screening process. The TSA is part of the Department of Homeland Security, which is the largest federal agency affected by the partial shutdown.

TSA spokesman Michael Bilello said Tuesday that “call outs” were slightly higher at 4.6 percent versus 3.8 percent at this time last year but that that the number was not large enough to have a significant impact on operations.

TSA officials have declined to detail the total number of screeners who aren’t showing up for work, saying that personnel who would provide those answers have been furloughed.

There have been scattered complaints about long lines, but for the most part, travelers say they aren’t seeing an impact. Many report security lines are moving and that at many airports, the checkpoints appear to be fully staffed.

But that could change after Friday, when most TSA employees are scheduled to be paid. No deal to end the shutdown means no paycheck.

For concerned travelers, the advice remains the same: Airlines and airport officials advise passengers to allow plenty of time to get through security.

And, if you have questions, the agency’s @askTSA Twitter handle is staffed and responding to questions, including whether screeners who are on the job even despite not being paid can accept tips. (No, they cannot.)

Note that those who apply for the Global Entry program may face delays since many appointments have been canceled due to the shutdown. However, TSA’s Precheck program is continuing to accept applications. Precheck is funded by user fees, so it is not affected by the shutdown.

Still, the uncertainty of what might happen should more TSA screeners call out is drawing attention from the Hill.

In a follow-up to a letter sent Tuesday, Rep. Bennie Thompson (D-Miss.), chairman of the House Homeland Security Committee, blasted DHS for failing to be “forthcoming about the security implications of President’s shutdown on DHS” and continued to press for answers on whether TSA is able to carry out security functions, particularly at foreign airports.

On Monday, Thompson sent a letter seeking answers from TSA Administrator David Pekoske about how many officers had failed to show up for work and whether TSA has a contingency plan in place to ensure the nation’s airports are secure. However, TSA officials said they could not provide written answers due to the shutdown.