From Scientific American:
The first bomb rocked Seaside Park, N.J., on Saturday morning as the Seaside Semper Five race was about to start. Several “pipe bomb-type devices” had been wired together and placed in a plastic garbage can, but the explosion did not cause any injuries. But at 8:30 p.m. a bomb placed in or on a Dumpster injured 29 people in New York City’s busy Chelsea area, where the blast damaged surrounding buildings and shattered nearby windows. Soon after that police discovered a second explosive device made using a pressure cooker a few blocks away. A mobile phone had been taped to the device, apparently as part of a remote detonation mechanism.
Still another incident rattled nerves Sunday evening, when two men found a backpack stuffed with as many as five IEDs near a train station in Elizabeth, N.J. One of the bombs detonated as police tried to disarm it with a robot. Law enforcement officials on Monday apprehended Ahmad Khan Rahami, a 28-year-old naturalized U.S. citizen of Afghan descent, in connection with the bombings.
A federal law enforcement official told The Associated Press that the Chelsea explosion left a residue of Tannerite—the brand name of a substance made up of ammonium nitrate and aluminum powder and designed to explode only when the two chemicals are combined and hit by a high-velocity bullet from a firearm. Tannerite, primarily used at firing ranges to create exploding targets, needs a relatively high-energy impact to set it off, and cannot be directly detonated with a simple flaming fuse or electrical signal from a cell phone alone.
New York Times correspondent Rukmini Callimachi reported Monday on Twitter that the second Chelsea bomb and at least one of the devices in Elizabeth contained hexamethylene triperoxide diamine (HMTD), an explosive organic compound. …