A month before the Boston Marathon bombings, the FBI warned that commercial "exploding targets" used for recreational rifle shooting could power homemade bombs on American streets.

Exploding targets like Tannerite, which consists of a mixture of ammonium nitrate and aluminum powder, are legal; available at sporting-goods stores and websites; and retail for fairly cheap. The official manufacturer sells ten 1-pound packs for $89.99. The FBI recently expressed concern that tighter restrictions on common bomb precursor materials like ammonium nitrate fertilizer could lead wannabe domestic extremists to pack their homemade bombs with the stuff.

"The FBI assesses with high confidence recreationally used exploding targets (ETs), commonly referred to as tannerite, or reactive targets, can be used as an explosive for illicit purposes by criminals and extremists and explosive precursor chemicals (EPCs) present in ETs can be combined with other materials to manufacture explosives for use in improvised explosive devices (IEDs)," the bureau warned on March 5. (.pdf)

To be clear, there is no evidence that Tannerite or a knock-off was the explosive used in the two Boston bombs that killed three and wounded over 170 on Monday afternoon. The FBI is set to analyze the bomb fragments and other trace elements from Copley Square at its explosives lab in Quantico, Virginia. But the bureau's explosive-targets warning underscores the longstanding concern that the homemade bombs used as signature insurgent weapons in Iraq and Afghanistan can be constructed in the United States with ease.

Underlying the FBI's concern: exploding targets don't meet the federal definition of an explosive, so "their purchase is not regulated to the same degree as ammonium nitrate-containing fertilizers." Tannerite, for instance, is sold in an inert, binary form, so it can't explode before you want it to. Shooting-sports enthusiasts simply mix up a canister of the stuff, place it on a firing range, and let off a round – the energy from a rifle shot causes a big explosion, verifying that a shooter successfully hit his or her target.

"Currently, anyone can purchase ETs without an FEL [federal explosives license] and have them shipped without expensive hazardous materials shipping fees or placards," states the March bulletin, which was first found by the Examiner and Public Intelligence.

The bulletin provides a series of circumstantial cases indicating that criminals and extremists might be interested in exploding targets, but not many specific or verified cases. "Law enforcement reporting" claims that in 2011, an "unidentified U.S. person in Missouri" threatened to detonate himself in a mayor's driveway using a homemade bomb "containing 20 pounds of material harvested from ETs."

But the point of a homebrewed bomb is that it's easy and cheap to construct using commonly available items. Statistics kept by the Pentagon's bomb squad, known as JIEDDO, showed that by 2011, the average price for homemade bomb components had fallen to $265, and an average of 621 IED explosions occurred monthly outside of Iraq and Afghanistan. Its most recent four-year plan, issued last year, warned that there is "no single solution to defeat the IED," since IED networks are diffuse, ad hoc and varied around the world, "from criminal gangs to insurgencies to terrorists with global reach." The Boston bombs are believed to have been constructed using pressure cookers that contained explosives, BB pellets and nails.

The United States is clearly a target for the homebrewed bombs, as JIEDDO's director has frequently warned. In 2010, for instance, would-be bomber Faisal Shahzad attempted to detonate an SUV in Times Square. The fear of a successful domestic homemade bomb threat prompted the White House to release a "counter-IED" strategy in February based around controlling precursor explosives, sharing technical countermeasures and coordinating responses between federal and state law enforcement.

Again, the FBI has yet to determine or disclose what explosives the Boston Marathon bombs used. But now that the domestic IED threat has manifested, the FBI is worried about what will fuel the next bomb. And lots of things can be used to construct the next bomb.