As a team of investigators led by the FBI begins deciphering the bombs that killed three people and wounded 150 more in Boston this week, a key clue is already in plain sight on countless videos taken during the blasts: the color of the smoke.
The color provides important insights into the type of explosive used in the blasts, which President Obama on Tuesday characterized as "an act of terrorism." Michael Marks has been watching those plumes as he views the photos and videos of the Boston Marathon attack. Marks retired from the Naval Criminal Investigative Service in August after analyzing bombings across the Mideast, including the 2000 attack on the USS Cole. The smoke color isn't clear, but the signals it sends are important.
Analyzing the color of the smoke can provide information about the explosive that powered the bombs, which in turn provides clues about its sophistication — and, possibly, that of the people who made it. Such insights will be key for the inquiry, one of many pieces of evidence considered during the forensic analysis by investigators. Different chemical compositions, when subjected to heat and pressure, produce the different colors of smoke.
"Bright white smoke, that's black powder," Marks tells Danger Room. By black powder, he means typical gunpowder, the type you can purchase from commercial ammunition or cook up — dangerously — at home using sulfur and other ingredients with recipes easily found via Google. "Dirty grey [smoke], that's high-explosive."