By Paige Williams, Elements blog @ newyorker.com, April 17, 2013

[.....] Hall said he was particularly intrigued, looking at pictures from the marathon scene, by the smoke. It was white, and immediate, and there was lots of it, which he said isn’t usually the case. “I was puzzled about that,” he said. Hall is thirty-five but has silvering hair, buzzed short— not unlike that of military guys or the law-enforcement officers he worked with early in his career—and now he ran a hand through it. To him, the color of the smoke, and the volume, suggested a high explosive.

Investigators would be asking witnesses not only what they saw and smelled (you don’t need to be a scientist to know that sulfur smells different than gasoline) but also what they heard. They would be listening to the blasts themselves, via valuable video footage—law-enforcement authorities have put out a call for any and all images. The trained ear can evaluate the brisance, or sound of an explosion, the way an oenophile might be able to tell a Chardonnay from a Pinot Grigio from a Sauvignon Blanc, or a gun enthusiast might tell a cap gun from a handgun from a shotgun. “Higher explosions tend to give a very loud crack rather than a low-toned boom,” Hall said.

The investigators would also consider [....]