The events of Sept. 11 have ratcheted up security at American airports to the highest level ever, according to a spokesman for Transportation Secretary Norman Mineta. But to say there is plenty of room for improvement puts it mildly. Hundreds of employees with access to high-security areas at 15 U. S. airports have been arrested or indicted by federal law enforcement officials for using phony Social Security numbers, lying about criminal convictions0r being in the United States illegally. None of those arrested had terrorist links, but some aviation experts said the workers were in a position to help smuggle weapons or bombs aboard aircraft if they had wanted. Tests ordered by President Bush and conducted by federal agents at 32 airports between November and February, when airports were on highest alert, showed that security screeners failed to detect knives 70% of the time, guns 30% of the time and simulated explosives 60% of the time. Two members of the House Transportation Committee are pushing to reverse the administration's opposition to arming pilots because groups representing pilots are insisting that their members need to be armed as a last line of defense. Attorney General John Ashcroft said the arrests of hundreds of airport employees showed that the system of background checks--done piecemeal by airlines, private contractors and others--needs tightening. That much is painfully obvious. What isn't clear is why the system was so porous (有漏洞的) to begin with and why it wasn't immediately tightened after that infamous Tuesday in September. Some people in the industry wisely have suggested that all airport workers be required to pass through the same metal detectors and other security checks as flight crews do, Congress has ordered the new Transportation Security Administration to find ways to enact just such a requirement. Unfortunately, no deadline has been set, in part because federal officials are preoccupied with getting thousands of new baggage screeners in place by Nov. 19--when the feds take over airport security--and installing bomb-detection equipment in all airports by the end of the year. Plainly, those two goals are critical. But it would be a mistake to give low priority to fixing other gaping holes in the nation's airport security net. If the federal crackdown is going to be effective, it needs to be comprehensive. The possible reason for hundreds of airport employees being arrested might be one of the following except ______.