None of the dozen pipe bombs sent to prominent critics of President Trump this week detonated, and the FBI has so far refused to identify what — if any — explosive substances they may have contained. But ABC News has reported that the device sent to George Soros’s home contained an “explosive black powder,” and CBS News has likewise reported, citing a law enforcement official, that at least two of the devices contained a “black powder.”

On Friday, the FBI arrested and charged a Florida man for carrying out the attempted bombing campaign, and the investigation is ongoing. If confirmed, the use of black powder (also known as gunpowder) would fit a pattern: It was the third most commonly used explosive charge in domestic bombing incidents last year, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives. Black powder (or its modern cousin, smokeless powder) has been used in countless high-profile bombings, including the World Trade Center in 1993, Atlanta in 1996, Boston in 2013, and the Unabomber’s two-decade-long spree.

It’s also untraceable in the United States, thanks in large part to the National Rifle Association’s 50-year campaign against regulations that would require the addition of trace markers, or “taggants.” The minuscule, indestructible chips can make it possible to link gunpowder back to the point of sale — information that can help investigators quickly track down the bomber.

But because many gun enthusiasts use gunpowder to make or reload their own bullets, the NRA has been successfully killing efforts to add taggants since the Nixon administration, according to a report released Thursday by the Violence Policy Center. The NRA has offered a host of reasons over the years for its opposition: There wasn’t enough credible research on their efficacy; taggants could destabilize gunpowder and affect the trajectory of bullets; adding them would be too costly for manufacturers; and implementing a tracing system would be too costly for taxpayers.

But critics of the NRA say the group’s opposition to taggants is rooted in the same rationale that animates its opposition to universal background checks and a centralized database of gun sales: Because they could be a backdoor to national gun registration.

According to the Violence Policy Center report, the NRA first began its fight for gunpowder rights in March of 1970, when, in the middle of a wave of political bombings, the Nixon administration moved to regulate explosive materials. At a House Judiciary subcommittee meeting that July, then-NRA President Woodson Scott testified that regulating gunpowder would “place unnecessary burdens” on gun owners. When the bill passed, it exempted most smokeless powders from federal regulation, and included an exemption for “black powder in quantities not to exceed five pounds.” Three years later, the NRA successfully lobbied to expand that exemption to up to 50 pounds.

Taggants entered the picture soon after. The ATF launched a $5 million pilot program in 1977 in which four manufacturers of dynamite and liquid explosives added a taggant manufactured by 3M to their products. Trace markers led to the apprehension of a truck bomber in 1979.

But that same year, according to The New York Times, the NRA successfully pressured Congress to halt all research into taggants. After a 1980 congressional report found that taggants could increase bombing arrests by as much as 75 percent, the NRA pushed back, insisting that they weren’t safe.

Again, the NRA prevailed. A House Appropriations subcommittee voted to prohibit the ATF from funding research into the new technology — a foreshadowing of the 1996 appropriations rider that practically halted federally funded studies on gun violence. For the next 13 years, members of Congress inserted language into ATF appropriations bills stating that “none of the funds appropriated herein shall be available for explosive identification or detection tagging research, development, or implementation.” During that same time period, the number of bombings involving black and smokeless powders nearly doubled.

The Oklahoma City bombing prompted both the White House and Congress to revisit taggants. An ATF supervisor told The Los Angeles Times at the time that taggants “would have been very valuable” in investigating the 1995 bombing. According to the Violence Policy Center report, another ATF official wrote that unregulated smokeless and black powder “represent one of the greatest threats to the American public posed by bombers.” In the wake of the attack, Wayne LaPierre, then four years into his tenure as the public face of the NRA, even said the group would “take a whole other look” at the use of taggants in commercial explosives.

The 1996 Antiterrorism and Effective Death Penalty Act called for studies into the feasibility of adding taggants to gunpowder and explosives. Manufacturers have been open to the practice; some voluntarily add another kind of trace markers — dyed particles — to help customers differentiate their products. But when the time came to actually study the materials, the NRA told researchers that taggants might affect the burning properties of smokeless and black powders, and anyway, they wouldn’t prevent bombings.

But taggants could help law enforcement identify serial bombers before they strike again. Without tracing technology, it took authorities two years to catch Eric Rudolph, whose pipe bombs filled with gunpowder and nails resulted in two deaths and more than 100 injuries at the 1996 Summer Olympics in Atlanta. And he was only apprehended after ATF agents interviewed the Tennessee gun dealer from whom he’d bought smokeless powder — 50 pounds of it, which is just under the higher federal reporting limit that the NRA had fought for. In the meantime, Rudolph bombed two abortion clinics and a lesbian bar, attacks that killed two more people and injured more than 50 others.

The chief architect of the 2013 Boston Marathon bombing bought two firecracker kits containing a pound and a half of black gunpowder from a fireworks store in New Hampshire. But because the gunpowder didn’t contain taggants, investigators had one less tool to solve that devastating attack.

The NRA did not respond to an email asking if its position had changed.