It depended on how fast new screening machines can be built, she said.

The project will also probably use an island-style arrangement instead of a wall of counters in the terminal to accommodate bulky new baggage screening machines.

The performance of the screening machines is a loosely guarded secret.

The airports will require about 2,000 explosive screening machines.

In some cases, the screening machines, which are the size of minivans, sit in temporary locations, as workers prepare permanent sites with adequate power and access to conveyors.

Officials acknowledge, moreover, that the screening machines can generate false positive readings for explosives up to 30 percent of the time.

The old screening machines were designed to stop 20th-century terrorists.

Transportation officials also do not specify whether the inspections should be done by opening a box, using a screening machine or a dog, or other means, leaving the airlines to decide.

New technologies like robotic screening machines that rapidly test millions of chemical compounds against biological targets have not worked as well as industry executives hoped.

Flight crews in uniform currently bypass the screening machines by showing ID cards.