Patrick Gannon, police chief of Los Angeles International Airport, said the 54,000 employees with security badges undergo recurring criminal background checks and random screening. But even that may not be enough.
"I agree that in any airport throughout the United States and here also, there is never a 100% guarantee that somebody who wanted to do something illegal or wrong couldn't make that happen," Gannon said.
Only two major airports in the United States -- Miami and Orlando -- conduct full employee screening by requiring all employees with access to secure areas to pass through a metal detector, a CNN investigation found.
Other airports like Los Angeles conduct random screenings. Many airports don't do any screenings at all other than a criminal background check before employees are hired.
The crackdown on employee screening comes after the arrest of a Delta baggage handler and passenger accused of gun smuggling at Atlanta's Hartsfield Jackson International Airport in December.
Both Atlanta and Los Angeles have since begun restricting access
by closing access doors that lead to the airfield.
Nevertheless, Gannon acknowledged that the potential of a lone wolf committing a crime at the airport worries him.
"It concerns me all the time," he said. With so many employees, "[T]here is no way that you are going to have the ability to screen every single person that comes to work in the airport."
At LAX, employees punch in a code and swipe their badge to access the airfield. Gannon said about 18 out of approximately 300 access doors have been closed as the airport evaluates employee screening procedures.
He said, "the more you limit the access doors, the more you can focus things like spot checks and screenings and cameras."
Even at Miami International Airport, Gannon said, "they still can't 100% say that they can keep contraband or any type of item out of their restricted area."
Lauren Stover, Miami airport security director, agreed that ID badges alone "are not enough [to] stop malicious intent."
"You can vet employees for basic information on their backgrounds, but it's not going to prevent them from carrying out some kind of malicious activity against an airport. And airports need to know their threats. For us, we have threats from criminal activity to being targets of a terrorism attack," Stover said.
In the wake of congressional scrutiny of airport employee screening procedures, private security guards at Atlanta's airport last month began checking employee bags as the first step toward moving toward full employee screening. The airport also plans to reduce the number of secure access doors from 70 to 10.
"In the last six months ... people are being recruited to engage in terrorist acts," said Miguel Southwell, the airport's general manager. "People are being recruited from the United States. So now we have a greater insider threat."
The TSA is conducting a review of the feasibility of full employee screening, although previous government studies have concluded that would be too expensive and inefficient.