Airline terror warning prompted by Al Qaeda affiliate threat, sources say

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How serious is new shoe-bomb threat?

Homeland Security alerts airlines of potential risk

A security threat issued this week that warned airlines of shoe bomb attacks was prompted by heightened U.S. concerns about Al Qaeda's Yemen-based affiliate, senior law enforcement officials told Fox News.

The Department of Homeland Security issued the warning on Wednesday about U.S.-bound flights out of concern that Ibrahim Hassaon al-Asiri, a Saudi believed to have been behind previous failed attempts to attack planes, may have come up with new bomb designs that could evade airport security measures, the sources said.

Al-Asiri is considered by U.S. officials to be the best terrorist bomb-maker in operation today. He was behind the failed 2009 attempted bombing of an airliner over Detroit on Christmas Day by a Nigerian militant who had an explosive charge built into his underwear, as well as cargo printer bombs that had explosives packed in toner cartridges. Those bombs were intercepted. There has been credible intelligence in the past that al-Asiri has been training and sharing his bomb-making skills with others.

The bulletin called on screeners to increase their use of explosive detection swabs on passengers’ shoes. Fox News was told these very visible changes to security procedures are expected at airports overseas and not domestically, and passengers flying into the United States can also expect more pat-downs. Among the airports being advised to add extra security are Amsterdam's Schiphol airport, London's Gatwick Airport and Manchester Airport in England.

The warning was not related to the Winter Olympics in Sochi, Russia. Earlier in the month, federal authorities had warned about possible attempts by terrorists to use toothpaste tube-based bombs on flights between Russia and the United States.

Earlier Thursday, Jeh Johnson, the Homeland Security Department secretary, said that the latest warning was a routine advisory issued in response to the latest intelligence.

"As you know, concerns about shoe bombs have been out there for years," Johnson said during a news conference at Los Angeles International Airport. "Every once in a while we update our advisories, we modify our procedures, so we remain vigilant in dealing with the various potential threats that exist."

A U.S. intelligence official told The Associated Press that DHS released a notice to airlines reiterating that liquids, shoes and certain cosmetics were of concern. The warning was focused on international flights into the United States.

Johnson also commented on his decision Wednesday to withdraw a contract proposal asking a private company to give the government access to a nationwide database of license plate tracking information.

The proposal said Immigration and Customs Enforcement was planning to use the license plate data in pursuit of criminal immigrants and others sought by authorities.

"I think that any proposal of that nature should require a careful review as it concerns privacy, civil liberties concerns, at the senior levels of the department, including myself, so that's why I did it," Johnson said.

Johnson spoke to a crowd of reporters beyond the screening checkpoint area while flanked by six Transportation Security Administration officers. He emphasized multiple times in his opening remarks that he'd come to LAX to "express solidarity with my TSA colleagues." The agency lost its first officer in the line of duty during a Nov. 1 attack at the airport by a gunman targeting the agency's employees. Two other TSA officers and a traveler was wounded.

TSA is conducting a review of airport security policies as a result of the shooting. That's in addition to a separate investigation into the shooting the airport's expected to make public next month.

"I asked them (TSA officers) directly: 'Tell me what more we can do,'" Johnson said. He said he agreed with TSA Administrator John Pistole's decision to reject arming officers in response to the attack.

Fox News' Jake Gibson, Catherine Herridge and The Associated Press contributed to this report.