Ever wanted to go back to college for the day? Don’t miss: 3 top lecturers in Baltimore

Oklahoma City Bombing (1995)

A better way to track a bomber

A better way to track a bomber

The mayhem in Boston the week of April 15 was a reminder of how an American city can be paralyzed by a homemade bomb. The same kinds of improvised explosive devices that menaced U.S. troops in Iraq and Afghanistan can easily be deployed by freelance terrorists or madmen trying to send a message, incite panic or just create a media spectacle.

The Tsarnaev brothers were identified as suspects in the bombing because of surveillance videotape, but the FBI might have been able to do it faster if tiny plastic markers had been part of the small-arms propellant packed into the pressure-cooker bombs. These little chips, called "taggants," have been around for close to 40 years, and...