DENVER -- Denver International Airport began compliance Monday with a Federal Aviation Administration order to airports and airlines to re-do their background checks on employees.

7NEWS confirmed that at about 3 p.m. Monday, officials at Denver International Airport started re-validating all employees with security access.

During the next few days, DIA will review 23,000 employees.

The order from the FAA requires the airport to conduct new criminal background checks and reissue security badges for all baggage handlers, food service workers and any employee with access to planes, ramps, tarmacs or any secure location at the airport.

FAA also wants to make sure all airport IDs are genuine, current and belong to the person they are assigned.

DIA spokesman Chuck Cannon said Monday that about 23,000 identification badges will have to be re-certified. About 900 of them belong to employees of the city of Denver, the owner of the airport.

The remainder belong to employees of the airlines, security contractors and other employers operating at DIA.

Airports and airlines are required to do new criminal checks and scrutinize employment histories for baggage handlers, food service workers and other employees who have access to airliners, ramps, tarmacs and other secure areas.

The FAA directive came after suspicion that someone other than the hijackers planted box-cutting tools aboard the commandeered planes. Several passengers of the hijacked planes said the terrorists were threatening passengers and crew with box cutters.

"It means the airport has to make sure the IDs are all current, are genuine and that they're tamper-proof and all difficult to counterfeit," said FAA regional spokesman Allen Kenitzer of Seattle.

He said the work must be completed as quickly as possible, but said there was no deadline.

Cannon said airport operations will continue while the work is being completed.

Much of the work will be done by United Airlines, which handles 67 percent of passengers at DIA, and its contractors. United spokeswoman Chris Nardella declined to comment on specific security measures, and said the company was not yet sure how long it would take to comply with the order.

United also is the dominant carrier at the Colorado Springs city-owned airport, where airport spokeswoman Mary Collins said about 90 airport employees would have to be re-certified. She said that work should be done within a week.

Passengers Say DIA Among Best At Improving Security

Travelers at DIA Monday told 7NEWS that they felt that Denver's airport was better at implementing tighter security than most other airports they'd been to.

A new security checkpoint stops incoming vehicles well before the parking area. Passengers are required to show an ID and ticket, and then are subject to random bag searches, 7NEWS reported.

At other airports, some passengers reported that security varies.

"Half the time they'd ask to see an ID, but the other half of the time they'd just ask for a boarding pass," passenger Sarah Burlingame, who had just arrived from the San Diego airport, said.

"Both in London and in the US, I think there was something about examining inside hand baggage, but nobody ever did," international passenger Andrew Solomon said.

Most everyone that spoke to 7NEWS said that they felt safe at DIA.