At some airports, passengers have been told they had to take off their shoes before passing through metal detectors; at others, it was optional.
The Transportation Security Administration now says removing shoes will be optional. However, passengers will be warned their chances of being asked to submit to a more thorough secondary screening will be greater if they do not take off their shoes and put them through X-ray machines.
"We must make sure our security process is consistent so air travelers know what to expect at every airport in the country," TSA chief James Loy said in a statement.
The TSA has found security lines move faster if everybody removes their shoes. Shoes with metal shanks or steel toes, for example, set off metal detectors and must then be screened with hand-held wands, slowing the process of clearing passengers into gate areas.
The agency said it has increased scrutiny of shoes in response to information gathered by intelligence agencies.
Loy said al Qaeda has trained people to use shoe bombs, as evidenced by Richard Reid's attempt to blow up a trans-Atlantic flight on Dec. 22, 2001.
Reid, a British citizen who said he was a member of al Qaeda, tried to light explosives in his shoes on a Paris-to-Miami American Airlines flight, but was overpowered by passengers and crew members.
He was sentenced to life in prison in February after pleading guilty to eight charges including attempted use of a weapon of mass destruction.