The Transportation Security Administration said it is unlikely to detect and unable to extinguish what an FBI report called “the greatest potential incendiary threat to aviation,” according to a classified document obtained by The Intercept. Yet despite that warning, sources said TSA is not adequately preparing to respond to the threat.

Thermite — a mixture of rust and aluminum powder — could be used against a commercial aircraft, TSA warned in a Dec. 2014 document, marked secret [PDF here]. “The ignition of a thermite-based incendiary device on an aircraft at altitude could result in catastrophic damage and the death of every person onboard,” the advisory said.

TSA said it is unlikely to spot an easy-to-assemble thermite-based incendiary device during security screening procedures, and the use of currently available extinguishers carried on aircrafts would create a violent reaction. The TSA warning is based on FBI testing done in 2011, and a subsequent report.

A thermite device, though difficult to ignite, would “produce toxic gasses, which can act as nerve poison, as well as a thick black smoke that will significantly inhibit any potential for in-flight safety officers to address the burn.”

TSA warned federal air marshals not to use customary methods of extinguishing fires — the water or halon fire extinguishers currently found on most aircraft — which would make the reaction worse, creating toxic fumes. Instead, air marshals are told to “recognize a thermite ignition” — but TSA has provided no training or guidance on how to do so, according to multiple sources familiar with the issue.

TSA circulated these Dec. 2014 materials through briefings, according to sources familiar with the issue, but did not offer up guidance on what to do with this information, and equipment that could mitigate this threat, like specific dry chemical extinguishers, has not been provided. According to the TSA advisory, federal air marshals and other on-flight officers should: recognize a thermite ignition, advise the captain immediately, ensure the individual who ignited the device is “rendered inoperable,” and move passengers away from the affected area.

“We’re supposed to brief our [federal air marshals] to identify a thermite ignition — but they tell us nothing,” said one current TSA official, who asked not to be named because the official is not authorized to speak to the press. “So our guys are Googling, ‘What does thermite look like? How do you extinguish thermite fires?’ This is not at all helpful.”

Several aviation officials, who also asked not be named, confirmed they had been briefed on the threat, but given no information or training on identifying thermite ignition. “They say to identify something we don’t know how to identify and say there is nothing we can do,” one federal air marshal said. “So basically, we hope it’s placed somewhere it does minimal damage, but basically we’re [screwed].”

Aviation security officials who spoke with The Intercept said TSA floods its employees with intelligence products from other agencies on various types of threats, but does not tell its employees what, if anything, to do about this threat. “You’re signing off on this saying you’ve received this briefing,” a former transportation security official said. “This covers their ass in case something happens, they can say, ‘We shared our intel.’”

“As a general matter, DHS, the FBI and other partners in aviation security regularly share information on potential threats affecting air travel safety,” S.Y. Lee, a spokesman for the Department of Homeland Security, told The Intercept in a statement. “This information is shared in a timely and consistent fashion. When relevant and actionable information is developed, we work to identify countermeasures to mitigate the threat.”

The TSA bulletin was distributed by the agency’s Office of Intelligence and Analysis in response to a Dec. 10 classified 29-page FBI Intelligence Assessment titled, “Threat Assessment of Viable Incendiary Devices to Passengers and Aircraft.” A copy of that report was also obtained by The Intercept.

According to the FBI’s description of their tests, thermite devices “spew molten metal and hot gasses” and can potentially “burn through steel and every other material” on the aircraft.

“Since 9/11/01, coordination between public safety, aviation, and national security focused agencies occurs on a continuous and collaborative basis to identify and neutralize threats to aviation safety,” the FBI said in a statement to The Intercept. “While often this analysis is done regardless of the specific threat environment, we nonetheless work closely with aviation security experts regularly to examine all potential vulnerabilities. Information is then shared immediately with those responsible for aviation security for appropriate security enhancement considerations.”

Despite the TSA warning, and FBI report, how much of a threat thermite represents to aviation security is up for debate.

“Available reporting at this classification level, however, does not indicate any extremist interest in thermite to target aircraft,” the FBI report said.

A source with knowledge of current threats to aviation said other intelligence gained from safe houses overseas points to greater interest, not in thermite, but in other types of incendiary materials.

Jimmie Oxley, a professor of chemistry at the University of Rhode Island, and an expert in explosives and explosives detection, said thermite — though a theoretical threat — seemed an unlikely candidate to slip through security, particularly since the would-be terrorist would also have to carry an igniter. “You’ve got to get a pound of something that is a really thick mass through security without anyone noticing,” she said. “I find that hard to believe.”

The problem is one of practicalities, said Oxley, who has worked with the FBI and other federal agencies on explosives testing, but was not aware of the specific TSA or FBI reports on thermite obtained by The Intercept. If the hope were to burn a hole through the aircraft, then the thermite would have to be placed on the floor, and then there’s still no guarantee it would take down the aircraft.

Setting off thermite is also impractical, according to Oxley. “Somebody has to give you time to play on the plane,” she said. “Like with the shoe bomber, people do notice if you’re doing something weird in this day and age.”

While declining to address thermite specifically, Lee, the Homeland Security spokesman, insisted that the aviation security system is robust: “Today, all air travelers are subject to a robust security system that employs multiple layers of security, both seen and unseen, including: intelligence gathering and analysis, cross-checking passenger manifests against watchlists, thorough screening at checkpoints, random canine team screening at airports, reinforced cockpit doors, Federal Air Marshals, armed pilots and a vigilant public. In combination, these layers provide enhanced security creating a much stronger and protected transportation system for the traveling public. TSA continually assesses and evaluates the current threat environment and will adjust security measures as necessary to ensure the highest levels of aviation security without unnecessary disruption to travelers.”

– Sharon Weinberger contributed to this article.

Photo: Paul J. Richards/AFP/Getty Images

* This article has been updated to include comment by the FBI that was provided to The Intercept after publication.