A test of an X-ray security scanner that can see through clothing and take clear pictures of the human body along with concealed weapons, has been delayed until early this coming year at Phoenix Sky Harbor International Airport.

Critics say the high-resolution images are too invasive, and the American Civil Liberties Union has called it a virtual strip search.

The federal Transportation Security Administration initially said one machine would be operating at Sky Harbor's Terminal 4 by Christmas.

However, the agency said technical difficulties were creating problems in setting up the equipment so it work properly with the airport's wireless connections.

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It also decided the holiday season was not a good time to start. "The last thing we wanted to do was start this during one of the busiest travel times of the year," said Nico Melendez, an agency spokesman.

He would not be specific about when the machine will be ready, saying only that it should be installed within a few months.

The TSA says it has found a way to adjust the machine's images so the normally graphic pictures can be blurred in certain areas while still being effective at detecting threats.

Others have said it could expose travelers to potentially harmful radiation, but the TSA said the scanner is designed to see only through fabric, not skin.

Sky Harbor will still be the first airport in the country to test the equipment, and the TSA will later test the machines at a handful of the country's other airports during 2007.

The technology already is being used in prisons and by drug enforcement agents, and has been tested at London's Heathrow Airport.

During the pilot program, the X-ray machine will be used only as a secondary screening measure; passengers who fail the standard screening process will be able to choose between the new device or a typical pat-down search.