WASHINGTON -- There are the lines, the scanners and the occasional need for a stranger to poke around in your luggage, but the No. 1 complaint passengers have with airport security is about removing their shoes.

Homeland Security Secretary Janet Napolitano said Tuesday that eventually that requirement will be dropped.

"You're going to see better technology over time" that will detect shoe bombs without running the shoes through the X-ray machine, she said.

But Napolitano wasn't able to say when new technology would come into use and she said restrictions on liquids are likely to continue.

"In terms of what we see coming in the months and years ahead, it will probably be easier ... to deal with the shoe issue before we can lift restrictions on liquids," she said at a breakfast hosted by Politico.

Transportation Security Administration officials have said that passengers who register for a trusted traveler program they are developing may be allowed to skip the shoe screening sooner than the general public.

Napolitano said restrictions on liquids are likely to continue in part because intelligence reports suggest that terrorists are still trying to use nonmetallic detonation devices aboard commercial airliners operating in the United States and Europe.

Millions of passengers in the United States take off their shoes at airport security lines every week because of one act: Three months after the 2001 attacks, British-born Richard Reid tried to set off a bomb in his shoe on an American Airlines flight from Paris to Miami. A self-proclaimed al-Qaeda member, he was subdued by the crew and passengers.

There hasn't been another shoe bomb attempt since, and aviation security experts question whether shoe removal is necessary.

"You don't take your shoes off anywhere but in the U.S. -- not in Israel, in Amsterdam, in London," said Yossi Sheffi, an Israeli-born expert on risk analysis at the Massachusetts Institute of Technology. "We all know why we do it here, but this seems to be a make-everybody-feel-good thing rather than a necessity."

John S. Pistole, the TSA administrator, cites a travel industry survey that found shoe removal was second only to the high price of tickets in passenger complaints.