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."

The photo above, taken shortly after the Copley Square blast, shows plumes of white smoke. This Boston Globe video shows a grayer color for the bomb in the foreground, and a darker color for the second bomb in the background.

"I'm seeing both dirty grey and white," Marks says.

That might be a trick of the light, a distortion of the imagery. Marks, who has 31 years experience in the Navy and local law enforcement, is anything but categorical. The imagery "tells me I'm too far away" from the scene, he says wryly. "In one explosion I saw both colors. That might not tell us anything."

Further analysis might. The type of explosive used dictates how much is used to produce the deadly effect seen in Boston on Monday, and could contain clues about the sophistication of the bomb-makers -- whomever they are. "You can kill yourself very easily" trying to homebrew high-explosives like acetone peroxide, commonly known as TATP, Marks said. The FBI is asking the public to tell law enforcement of any explosions they might have heard in remote areas that might indicate a "test" of the explosives.

The smoke color has been an indicator of powerful explosions in some of the most iconic and devastating terrorist attacks in the United States. The documentary history of the Murrah Building bombing in Oklahoma City in 1995 noted "a large black cloud of smoke" emerging; culprit Timothy McVeigh used a explosives made from fertilizer. Similarly, a survivor of the 1993 World Trade Center attack, which employed the class-A high explosive nitrourea (.PDF), recalled "greasy black smoke" filling his lungs.

Following up on news reports that circulated all day Tuesday, FBI special agent in charge Richard DesLauriers disclosed late Tuesday afternoon that what seems like a "pressure cooker device" formed the hull of the bomb, which may have contained "BBs [and] nails." The Department of Homeland Security warned in 2004 that bomb design was common in Afghanistan (.pdf), although insurgent bombs in recent years tend to avoid metal. (To be clear, that would not be evidence that the bomb is foreign: designs for such bombs are easy to find online.) Marks saw similar designs in Bahrain, where he analyzed black-powder bombs encased in fire extinguishers.

The pressurization, if true, is another potential clue about the bomb's explosive content.

"Black powder burns, it doesn't explode," Marks explains. "The explosion happens once the pressure builds up." At that point, the shell becomes "primary fragmentation," something adding to the shrapnel wounds reported by Boston hospitals treating the survivors.

The FBI-led investigation has asked Boston Marathon spectators to provide their photos and videos of the scene around Copley Square. But they may not need that crowdsourced footage to interpret the contents of the bombs. Explosives forensic investigations usually employ what's called a presumptive test kit -- you can buy one on Amazon -- to test the scene for residue, often using a chemical aerosol spray like Expray. The Bureau of Alcohol, Tobacco and Firearms, which is assisting the investigation, has tremendous experience with black powder, Marks says, from checking out "people blowing up themselves with pipebombs." DesLauriers said Tuesday that the collected evidence will be sent to the FBI explosive laboratory at Quantico, Virginia.

None of that resolves the harder question of identifying the bombmakers. The presumptive contents of the bombs -- pressure cookers, some kind of black powder, maybe some kind of high-explosives -- aren't easy to trace back to a purchaser, especially if the bombmakers didn't buy sufficiently vast quantities of precursor material to ping a law-enforcement watchlist. To find that out, investigators will have to follow different kinds of smoke signals.