The new head of Homeland Security played down a recent warning that terrorists might try to sneak explosives onto commercial planes in passenger shoes, saying the threat has been around "for years."
Law enforcement officials, speaking anonymously, said the alert was based on new intelligence indicating that a shoe bomb may be used to blow up a U.S.-bound jetliner.
But Johnson said that "concerns about shoe bombs have been out there for years. Every once in a while we update our advisories."
Transportation Security Administration officials said no new security procedures have been imposed on commercial travelers in response to the latest warning.
Johnson, who was confirmed to the post in December, was at LAX to tour the facility that was the site of a shooting in November that killed a TSA officer and wounded others.
Johnson said he opposes proposals to arm TSA officers and declined to comment on reports that two panic buttons designed to summon help at the screening gate were not operating on the day of the shooting.
In response to questions, he said he has asked his agency to withdraw a proposal to develop a license plate tracking system with access to data from commercial and private license plate readers.
"I think that any proposal of that nature should require a careful review as it concerns privacy and civil liberties," Johnson said.