LAS VEGAS — Shoe bombs remain a threat for US aviation, said Homeland Security Secretary Janet Napolitano, who said passengers will have to continue to remove their shoes at airport screenings.

Napolitano drew applause at a conference of travel and tourism industry leaders with her remarks that the department has been looking for a technological solution to the shoe problem.

"We're not there yet, so wear slip-ons," she said.

The shoe requirement is probably the most hated of a raft of security measures imposed on air travelers after the September 11, 2001 attacks.

Industry leaders have complained about a one-size-fits-all approach to aviation security that they say has discouraged travel and is costing billions of dollars in revenue.

Napolitano said officials are not yet able to ease up on those stringent measures.

"It's a technological problem," said Napolitano.

"Why? Because we know from a risk-based standard that our adversaries have tried before and are always attempting to see what they can get onto a plane that would constitute enough explosive material to blow a plane up," she said.

"That threat has not disappeared, and if anything even the public revelations out of the material seized out of this compound where (terror mastermind Osama) bin Laden was confirmed that aviation remains a target."

Napolitano said that nearly 10 years after September 11, the US government was pursuing strategies to take some of the hassle out of air travel, where possible.

She said nearly a million people have now enrolled in "Global Entry," a so-called "trusted traveler" program that allows pre-vetted passengers to clear customs more quickly. The administration's goal is to double that number.

Other changes have been to introduce express lanes for passengers with connecting flights.

She said 80 percent of travelers at the top 22 international airports in the United States now clear customs in less than 30 minutes.

"We are moving away from one-size-fits-all," Napolitano said.

"But it's a more difficult problem than you might imagine. It has to be done the right way, recognizing that it only takes one plane going down" to have a major economic impact, she said.