WASHINGTON — After spending millions of dollars testing four different scanning devices that would allow airline passengers to keep their shoes on at security checkpoints, the United States government has decided for now that travelers must continue to remove their footwear, by far the leading source of frustration and delays at the airport.

The Transportation Security Administration said it had rejected all four devices because they failed to adequately detect explosives and metal weapons during tests at various airports. One of the scanners is now used in airports in 18 countries.

Last September, Secretary Janet Napolitano of theHomeland Security Department raised hopes when she said that research and development on scanning machines was progressing and that air travelers would eventually be able to keep their shoes on.

But nearly a year later, the T.S.A., which is overseen by Homeland Security, said it was not any closer to finding a solution. Lisa Farbstein, a spokeswoman for the agency, would not address why it had rejected the devices.

Read more at NY Times