Transportation Security Administration screeners at Houston's Bush airport somehow managed to miss a loaded, high-powered Glock pistol in an empty computer carry-on bag.
Authorities said it happens all the time -- screeners asleep at the screen.
Results from the most recent tests by the government were apparently so bad, they are classified.
One source briefed on the latest results told ABC News the failure rate approached 70 percent at some airports. Two weeks ago, the new TSA director John Pistole told reporters that in some airports, every single test gun, bomb part or knife got past screeners.
Meantime, officials are investigating new terror threats from al-Qaeda. Captured insurgents in Iraq claim terrorists are planning suicide attacks in the U.S. and Europe over the Christmas holiday.
Officials say they have not yet seen evidence of specific plots aimed at the U.S.
The FBI and Homeland Security Department are alerting state and local law enforcement to be on the lookout for suspicious behavior and to regularly change security measures to interfere with any possible terrorist plans.
Interpol in Europe is also on alert.
The threats came after a suicide bomber blew himself up in Stockholm, Sweden, killing himself and injuring two people on Saturday.
Captured insurgents said that the suicide bombing was part of attacks being planned by al-Qaeda against the U.S. and Europe.
According to U.S. officials, there is specific intelligence on attacks being planned against Europe during the holiday season, but intelligence officials have not uncovered specific details of threats aimed at the U.S. Still, they cannot be ruled out, the officials said.
The Associated Press contributed to this story.