Detecting concealed weapons safely

How can detect concealed weapons in a manner which is not too obtrusive or damaging to Constitutional rights?

On July 24, 1998, a man entered the U.S. Capitol building in Washington, DC, with a .38-caliber handgun concealed under his clothing. A security check point with a portal weapons-detection system had been established at the entrance of the building. Knowing that his gun would be detected if he walked through the portal, the man stepped around it. Immediately, he was confronted by Jacob Chestnut, one of the Capitol Police officers operating the portal. The man drew his gun and killed Chestnut. He then shot and killed a second officer, John Gibson, before he was stopped.[1]

Seven years later, on December 5, 2005, a man with a bomb vest under his clothing approached a shopping mall in Netanya, Israel. His behavior alerted police and mall security. When he was confronted outside the mall, the suicide bomber detonated his bomb, killing 5 people and injuring 50.[2]

Although there has yet to be a suicide bombing in this country, such an attack could happen anywhere—on a bus, at a mall, at the Super Bowl, or at the Academy Awards. It is vital for law enforcement to be able to detect and respond to weapons at a sufficient distance to allow officers to make decisions and take actions that deal safely with the situation. For over a decade, the National Institute of Justice (NIJ) has been working to address this need.