Errors by screeners--not random computer glitches that the federal government previously blamed--were responsible for false alarms over weapons that sparked the recent evacuation of Midway Airport and two other U.S. airports, the Transportation Security Administration said Monday.
The acknowledgment may bolster Mayor Richard Daley's demand that the security agency reimburse Midway and the Chicago Police Department for costs associated with clearing about 500 passengers out of the terminal last week.
The expenses due to the screening lapse are still being tallied, said city Law Department spokeswoman Jennifer Hoyle, adding it would be the first time the city has tried to be compensated for mistakes made by the security agency.
The confusion that led to the full-scale terminal evacuation on Nov. 15 was prompted by a hand grenade appearing on an X-ray scanner. The image of the grenade, part of an exercise used to test screeners, should have been stored in a computer file by a security agency staff member as part of standard procedure before an employee shift change at the screening checkpoint, said Amy von Walter, spokeswoman for the security agency.
"We still intend to collect" payment to cover the costs of the evacuation, said Annette Martinez, spokeswoman for the Chicago Department of Aviation.
Federal security officials initially said a malfunction in a software program used to test screener performance prompted a computer-generated image of the grenade to appear randomly on the X-ray screen. A screener operating the X-ray scanner thought the grenade, artificially projected inside a carry-on bag, was real.
If the screener were being tested, the grenade image would have disappeared when the screener tapped a button on the device's console to acknowledge seeing the item. In this case, the grenade did not vanish.
But the passenger was able to leave the security checkpoint with the suspect bag before screeners could search its contents, leading to the evacuation order.
The incident also raised questions about how a passenger whose bag had been targeted for a search was able to leave the security checkpoint.
"It's inexcusable for a bag to walk off," said Kerry Spaulding, a former security official at the Federal Aviation Administration. "The X-ray operator has the obligation to control that bag until it is cleared."
Several days before the Midway incident, images of guns popped up on X-ray screens at Miami International Airport and at Dulles International Airport near Washington, D.C. In each case, passengers were told to leave terminals, and flights were delayed while security officials searched concourses for the non-existent weapons.
An investigation concluded human error was responsible in all three cases, von Walter said.
The grenade and guns were not computer-generated, but were items detected inside baggage on earlier occasions. Security managers saved the images of the weapons in a computer archive for possible training and periodic testing to check that screeners are paying attention.
Such testing was being conducted at Midway on Nov. 15, and screeners or security agency managers failed to close down the archived image during a shift change, von Walter said.
"Images of weapons that are kept in an archive should have been cleared out before the next screener went on duty," von Walter said. "That didn't happen."