TSA officer Robert Howard signals an airline passenger forward at a security checkpoint at Seattle-Tacoma International Airport in Washington on Jan. 4, 2010. Flight attendants, pilots, federal air marshals, and insurance companies are part of a backlash to the Transportation Security Administration’s policy, announced in March 2013, to allow onto planes passengers carrying small knives and sports equipment such as souvenir baseball bats and golf clubs. Elaine Thompson/AP
Transportation Security Agency (TSA) workers carry out security checks at Denver International Airport, the day before the Thanksgiving holiday in 2010. Rick Wilking/Reuters
An unidentified passenger participates in a full-body scan at the El Paso International Airport in Texas on Wednesday, Nov. 24, 2010, a busy holiday travel day. Mark Lambie/The El Paso Times/AP
Confiscated items are displayed during a news conference at the TSA training center at Newark Liberty International Airport, Nov. 17, 2011, in Newark, N.J. Julio Cortez/AP
A TSA worker (l.) runs her palms across a traveler's chest during a pat-down search at Denver International Airport in 2010. Rick Wilking/Reuters
A traveler looks at abandoned items at the Los Angeles International Airport in August 2006. Airport security passed along to passengers a prohibited item list that included liquids and gels, beverages, perfume and cologne containers, shampoo, creams, toothpaste, hair gels, and other items of similar consistency. Stefano Paltera/AP
Among the items confiscated from passengers at T.F. Green Airport in Warwick, R.I., are a disarmed grenade (c.) and two cologne bottles, left and right, shaped like grenades. The items, on display during a 2003 press conference, were confiscated since the previous November. Stew Milne/AP
A Transportation Security Administration officer checks a traveler's identification card at a security checkpoint at Baltimore-Washington International Thurgood Marshall Airport in Linthicum, Md., March 1, 2013. Patrick Semansky/AP
Labrador puppies Hoey (l.) and Hatton, named in honor of 9/11 attack victims Patrick Hoey and Lenny Hatton, who died in the World Trade Center, are pictured on the grounds of the Pentagon near Washington, June 28, 2011. The dogs are part of the Transportation and Security Administration's Puppy Program, in which young dogs are raised to become bomb-sniffers at air cargo facilities nationwide. Jason Reed/Reuters
A TSA officer waits by an X-ray machine at Los Angeles International Airport on March 4, 2013. Mario Anzuoni/Reuters
The Department of Homeland Security this week warned airlines to be on the lookout for shoe bombs on US-bound flights. Intelligence points to a possibility that new shoe-bomb technology may be out there.
US warnings to airlines to be on the lookout for shoe bombs are either a routine "update" or a worrisome sign that Al Qaeda operatives are at work on new explosives, depending on which report one reads.