Since the uncomfortably close attempted attack on Northwest Flight 253 last week, many have focused on why the alleged terrorist, Umar Farouk Abdulmutallab, was not placed on a watch list that would have prevented him from flying, even though the government had received information that he was a potential extremist. We should focus on a more fundamental question: How can we keep explosive materials off planes? Most airport security checkpoints use metal detectors. Al-Qaida has shown that it knows how to avoid detection by using an explosive device that contains little or no metal, such as PETN, or pentaerythritol tetranitrate, used by Abdulmutallab and 'shoe bomber' Richard Reid in 2001. During my time as secretary of homeland security, the Transportation Security Administration began working to replace the 1970s-era metal detectors used at airports across America with modern technology able to detect non-metal weapons concealed by terrorists on their bodies _ even in their underwear, where Abdulmutallab allegedly hid his bomb. The latest versions of these machines _ sometimes called whole-body imagers _ are deployed at 19 airports, and the TSA is attempting to place them throughout the nation. From the onset, deployment of the machines has been vigorously opposed by some groups. In June, the House of Representatives passed a bill that would prevent the TSA from using the new systems in most cases. If the House bill were to become law, the TSA would be limited to using the new technology only after a passenger had been selected for additional scrutiny. The vast majority of passengers would still pass only through metal detectors. So, under the standards set by the House bill, a terrorist not on a 'no-fly' list or a watch list mandating closer scrutiny _ like Abdulmutallab _ could probably carry a concealed non-metal weapon onto a plane undetected. Congress should reject this restrictive bill and instead fund a large-scale deployment of next-generation systems. Opposition to whole-body imagers essentially relies on three arguments. First, the American Civil Liberties Union and privacy advocates have complained that the machines subject passengers to a 'virtual strip search.' Second, they claim that the machines are unsafe because they expose passengers to dangerous amounts of radiation in screening. Third, some critics argue that the only correct approach to airline security lies in better intelligence. All of these objections lack merit. The 'safety' concern is particularly specious, because the technologies expose people to no more radiation than is experienced in daily life. The case of Abdulmutallab shows that we cannot simply 'rely on intelligence.' Abdulmutallab was not on a watch list that required closer scrutiny. Even if the review President Obama has ordered closes a gap that would have put Abdulmutallab or others on more select watch lists, there are plenty of terrorists out there about whom we know nothing. Too many potentially dangerous people simply would not appear on any watch list. We cannot put all our eggs in the 'intelligence basket.' That's why, since Sept. 11, 2001, we have worked to establish multiple layers of defense to protect the American people. Watch lists surely are an important layer, as is intelligence-sharing, but others, such as the deployment of advanced detection technology, are just as important. Claims that the screening amounts to 'virtual strip searches' is calculated to alarm the public. As if screening is meant to reveal people's private parts to TSA officers. But the agency has nonetheless taken privacy concerns seriously in creating procedures for using this technology. In deploying the machines, the TSA has strictly limited the number of officers who see the images; separates the officers looking at images from the passengers being screened (so the officers do not know which passengers the images belong to); and uses software to blur the faces on the images _ further protecting the anonymity of passengers. Moreover, the machines are configured to prevent TSA officers from storing or retaining any images. As an additional measure, passengers can choose not to walk through one of the machines and receive a physical examination instead. In short, the TSA has listened to the reasonable concerns of privacy advocates and incorporated numerous suggestions into its protocols to draw the right balance between security and privacy. The administration must stand firm against privacy ideologues, for whom every security measure is unacceptable. Failing to use all available tools to plug a gap in security puts the lives of airline travelers needlessly at risk. The writer was secretary of homeland security from 2005 to 2009 and is co- founder of the Chertoff Group, a security and risk-management firm whose clients include a manufacturer of body-imaging screening machines. 