U.S. homeland security officials are working to address aviation security vulnerabilities that come from more than 900,000 people whose jobs allow them largely unfettered access behind the scenes at the nation's airports.
Trying to address the so-called insider security threat could take on more urgency if the crash of a Russian airliner in Egypt's Sinai is proved to be the result of a bomb planted by someone with access to the plane.
Some U.S. national security officials, citing intelligence data, say there's a growing likelihood a bomb brought down the plane, and that it is likely the attackers took advantage of security gaps at the Sharm el-Shiekh airport to sneak the device on to the plane, though other causes have not been ruled out. Investigators say they haven't recovered physical evidence to corroborate suspicions of a bombing, and other possible causes haven't been ruled out.
While the U.S. has spent billions of dollars beefing-up screening of passengers with scanners and background checks, some top U.S. security officials worry about gaps in how airport workers are vetted.
The worries in the U.S. lie partly in the fact that the Transportation Security Administration, which oversees air travel security, relies on the operators of the nation's more than 450 airports to do the vetting of aviation workers. The airports use TSA contractors to do background checks, including checking terrorism databases, legal immigration status and criminal histories.
A U.S. official with knowledge of American aviation security and its vulnerabilities says that while U.S. security is viewed as the gold standard, the screening of workers poses cause for worry.
"(The TSA) checkpoint is only one part of it. You can lock that front door all you want, if you've left the back window open it doesn't really matter," the official said.
Concerns about a potential insider threat have prompted the British and Russian governments to send security experts to examine procedures at Egyptian airports, and Department of Homeland Security Secretary Jeh Johnson announced on Friday enhanced security measures will be instituted in a number of airports with direct flights to the U.S.
A source with knowledge of the measures confirmed to CNN's Rene Marsh that three of those airports are in Cairo, Kuwait City and Amman, Jordan."I want people to know that their aviation security officials working on their behalf are continually evaluating threats, potential threats, and we make adjustments all the time based on what we see," Johnson told CNN's Barbara Starr in an interview Saturday.
A Homeland Security Department inspector general report issued in June included this concern: "TSA lacked effective controls to ensure that aviation workers did not have disqualifying criminal histories and that they possessed lawful status and the authorization to work in the United States."
The inspector general's office conducted testing of the security checks on a sampling of airport workers and found 73 background checked-workers who got through the security vetting but should have been flagged for terrorism-related categories.
The TSA said these were people employed by airlines, vendors or other employers and that the problems in their backgrounds weren't flagged because TSA isn't authorized to receive all categories of terror watch-lists.
Part of the problem is that the TSA isn't allowed to decide what criteria counts as disqualifying when vetting airport workers.
"TSA is imprisoned by the statute that it's not up to TSA to determine what's a disqualifying offense. That is the blinding gap in the system here. The airports who have a vested interest in getting people on board are responsible for a good chunk of aviation security," according to one of the U.S. officials.
DHS and TSA officials referred CNN to comments from TSA Administrator Peter Neffenger from earlier this month when asked about the security issues.
Neffenger told a House hearing that the number of people referred to in the inspector general report was 69, not 73 and "they were not actually on the terrorist -- any terrorist watch list."
"That information wasn't sufficient to raise to known or suspected terrorist status," he said.
But Neffenger acknowledged there are problems with the vetting of airport workers, saying, "There's work to be done there."
He pointed to new measures announced in April, including more random on-the-spot screening of workers and a reduced number of access points to sensitive airport areas, telling Congress the agency is still in the process of implementing them.
The official who spoke to CNN about security vulnerabilities said the checks on airport workers is about the same as that for passengers who qualify for the TSA Precheck program, which typically allows passengers to board by walking through metal detectors instead of more invasive screening machines.