Catching terrorists carrying bombs still a tough problem to solve.

* When police in Watertown, Mass., honed in on Boston Marathon bombing suspect Dzhokhar Tsarnaev hiding in a boat stored in a backyard in April, they had several tools familiar to service members who fought in the Iraq and Afghan wars at their disposal.

A Star Safire High Definition sensor manufactured by FUR Systems mounted on a helicopter was able to provide authorities with an image of Tsarnaev under a tarp. An iRobot Packbot was then sent into for reconnaissance before police surrounded the boat and apprehended the suspect.

Missing from the equation is a technology that may have prevented the bombing--stand-off detection for person-borne improvised explosive devices.

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The problem is that there is nothing commercially available for police departments or other security entities that can scan a crowd, pick up the signatures of hidden explosives and alert officials to the presence of a bomb, said Jimmie Oxley, co-director of the Department of Homeland Security's center of excellence for explosives detection, mitigation and response at the University of Rhode Island.

There are companies that are working on the problem, she said. Suicide bombers are a particular concern because by the time they reach a checkpoint, it is too late. They can still claim many victims.

There are sensors that use various methods to detect smudges of explosives on clothing from distances, or a plume of chemical vapors emitting from a backpack, such as those allegedly carried by the Tsarnaev brothers.

"The problem is that each of those techniques has issues," Oxley said.

One example is Raman-based spectroscopy systems, which use lasers to identify the chemical composition of items from a distance. They have performed well in field tests, but it is not safe for eyes. That is unacceptable in applications where it scans crowds for tell-tale signs of explosives, she said.

"You need a system where you can...