Federal officials said today that they are expanding tests to speed up airport lines and improve security.

In a first in the U.S., the Transportation Security Administration said that it plans to begin using computed-tomography or CT scanners to inspect carry-on bags at one checkpoint in the Phoenix airport by the end of the year.

CT scanners are already used to screen checked baggage. The process is mostly automated — the scanners generate 3-D images that are analyzed by computers. Security workers only check a bag if something is suspicious.

The use of CT technology at airport checkpoints would eliminate the need for screeners to examine X-ray images of every bag.

TSA said it will work with American Airlines to make other changes to speed up screening this fall in Chicago, Dallas, Los Angeles and Miami.

The tests at those four airports mirror a pilot program that TSA is running with Delta Air Lines in two lanes at the Atlanta airport. There, suspicious bags can be sent to a separate area for more screening while other bags on the conveyer belt keep moving. Bins are automatically sent back to the head of the line, and they are given radio-frequency-identification tags for better tracking.

The agency said the scanners and other steps will cut the time travelers spend in line by about 30 percent. Long checkpoint lines have become a nuisance for travelers. Security experts warn that the long lines create targets for terrorists that are outside security checkpoints.